わからない。

以下、rubocop 実行時に遭遇したケースを強引に再現。
別途 rspec 実行時にも遭遇したので、rubocop 固有の問題ではないはず。

Alpine Linux 3.7 (musl) のスレッドスタックサイズが glibc と比べて小さい(80k)事で、Ruby 2.4.3 では発生しないが Ruby 2.5.0 では発生する、という実行環境固有の問題と考えるべきだろうか。

musl libc - Functional differences from glibc

ruby:2.5.0-alpine3.7

[docker-for-mac]% docker container run --rm -it ruby:2.5.0-alpine3.7 sh

# gem install rubocop
# cat <<\RUBY > sample.rb
require 'rails_helper'

RSpec.describe MyClass do
  describe '#my_method' do
    subject { MyClass.new.my_method(arg) }

    context 'when ...' do
      context 'when ...' do
        context 'when ...' do
          context 'when ...' do
            let(:arg) { create(:arg, :trait, attr: x) }

            it 'should be ...' do
              YourClass.new(
                x: X.x(
                  y: Y.y(
                    z: Z.z(
                      a: A.a(
                        b: B.b
                      )
                    )
                  )
                )
              )
            end
          end
        end
      end
    end
  end
end
RUBY
# rubocop sample.rb
Inspecting 1 file


0 files inspected, no offenses detected
Traceback (most recent call last):
        264: from /usr/local/bundle/bin/rubocop:23:in `<main>'
        263: from /usr/local/bundle/bin/rubocop:23:in `load'
        262: from /usr/local/bundle/gems/rubocop-0.52.1/bin/rubocop:12:in `<top (required)>'
        261: from /usr/local/lib/ruby/2.5.0/benchmark.rb:308:in `realtime'
        260: from /usr/local/bundle/gems/rubocop-0.52.1/bin/rubocop:13:in `block in <top (required)>'
        259: from /usr/local/bundle/gems/rubocop-0.52.1/lib/rubocop/cli.rb:38:in `run'
        258: from /usr/local/bundle/gems/rubocop-0.52.1/lib/rubocop/cli.rb:78:in `execute_runners'
        257: from /usr/local/bundle/gems/rubocop-0.52.1/lib/rubocop/cli.rb:150:in `execute_runner'
         ... 252 levels...
          4: from /usr/local/bundle/gems/rubocop-0.52.1/lib/rubocop/ast/node.rb:172:in `each'
          3: from /usr/local/bundle/gems/rubocop-0.52.1/lib/rubocop/ast/node.rb:174:in `block in each_child_node'
          2: from /usr/local/bundle/gems/rubocop-0.52.1/lib/rubocop/cop/variable_force.rb:102:in `block in process_children'
          1: from /usr/local/bundle/gems/rubocop-0.52.1/lib/rubocop/cop/variable_force.rb:369:in `scanned_node?'
/usr/local/bundle/gems/rubocop-0.52.1/lib/rubocop/cop/variable_force.rb:369:in `any?': stack level too deep (SystemStackError)

ruby:2.5.0-stretch

[docker-for-mac]% docker container run --rm -it ruby:2.5.0-stretch sh

# gem install rubocop
# cat <<\RUBY > sample.rb
require 'rails_helper'

RSpec.describe MyClass do
  describe '#my_method' do
    subject { MyClass.new.my_method(arg) }

    context 'when ...' do
      context 'when ...' do
        context 'when ...' do
          context 'when ...' do
            let(:arg) { create(:arg, :trait, attr: x) }

            it 'should be ...' do
              YourClass.new(
                x: X.x(
                  y: Y.y(
                    z: Z.z(
                      a: A.a(
                        b: B.b
                      )
                    )
                  )
                )
              )
            end
          end
        end
      end
    end
  end
end
RUBY
# rubocop sample.rb
Inspecting 1 file
.

1 file inspected, no offenses detected

ruby:2.4.3-alpine3.7

[docker-for-mac]% docker container run --rm -it ruby:2.4.3-alpine3.7 sh

# gem install rubocop
# cat <<\RUBY > sample.rb
require 'rails_helper'

RSpec.describe MyClass do
  describe '#my_method' do
    subject { MyClass.new.my_method(arg) }

    context 'when ...' do
      context 'when ...' do
        context 'when ...' do
          context 'when ...' do
            let(:arg) { create(:arg, :trait, attr: x) }

            it 'should be ...' do
              YourClass.new(
                x: X.x(
                  y: Y.y(
                    z: Z.z(
                      a: A.a(
                        b: B.b
                      )
                    )
                  )
                )
              )
            end
          end
        end
      end
    end
  end
end
RUBY
# rubocop sample.rb
Inspecting 1 file
.

1 file inspected, no offenses detected

--- 2.4/alpine3.7/Dockerfile    2017-12-28 20:34:43.000000000 +0900
+++ 2.5/alpine3.7/Dockerfile    2017-12-28 20:34:43.000000000 +0900
@@ -7,9 +7,9 @@
                echo 'update: --no-document'; \
        } >> /usr/local/etc/gemrc

-ENV RUBY_MAJOR 2.4
-ENV RUBY_VERSION 2.4.3
-ENV RUBY_DOWNLOAD_SHA256 23677d40bf3b7621ba64593c978df40b1e026d8653c74a0599f0ead78ed92b51
+ENV RUBY_MAJOR 2.5
+ENV RUBY_VERSION 2.5.0
+ENV RUBY_DOWNLOAD_SHA256 1da0afed833a0dab94075221a615c14487b05d0c407f991c8080d576d985b49b
 ENV RUBYGEMS_VERSION 2.7.4
 ENV BUNDLER_VERSION 1.16.1

gcc:6.3

[docker-for-mac]% docker container run --rm -it gcc:6.3 bash

# curl -s https://cache.ruby-lang.org/pub/ruby/2.5/ruby-2.5.0.tar.xz | tar -xJf -
# cd ruby-2.5.0
# ./configure && make && make install
# gem install rubocop
# cat <<\RUBY > sample.rb
require 'rails_helper'

RSpec.describe MyClass do
  describe '#my_method' do
    subject { MyClass.new.my_method(arg) }

    context 'when ...' do
      context 'when ...' do
        context 'when ...' do
          context 'when ...' do
            let(:arg) { create(:arg, :trait, attr: x) }

            it 'should be ...' do
              YourClass.new(
                x: X.x(
                  y: Y.y(
                    z: Z.z(
                      a: A.a(
                        b: B.b
                      )
                    )
                  )
                )
              )
            end
          end
        end
      end
    end
  end
end
RUBY
# rubocop sample.rb
Inspecting 1 file
.

1 file inspected, no offenses detected

gcc:6.4

[docker-for-mac]% docker container run --rm -it gcc:6.4 bash

# curl -s https://cache.ruby-lang.org/pub/ruby/2.5/ruby-2.5.0.tar.xz | tar -xJf -
# cd ruby-2.5.0
# ./configure && make && make install
# gem install rubocop
# cat <<\RUBY > sample.rb
require 'rails_helper'

RSpec.describe MyClass do
  describe '#my_method' do
    subject { MyClass.new.my_method(arg) }

    context 'when ...' do
      context 'when ...' do
        context 'when ...' do
          context 'when ...' do
            let(:arg) { create(:arg, :trait, attr: x) }

            it 'should be ...' do
              YourClass.new(
                x: X.x(
                  y: Y.y(
                    z: Z.z(
                      a: A.a(
                        b: B.b
                      )
                    )
                  )
                )
              )
            end
          end
        end
      end
    end
  end
end
RUBY
# rubocop sample.rb
Inspecting 1 file
.

1 file inspected, no offenses detected