ThoughtWorks and ThoughtBot are different companies, of which TW is well-known due to Martin Fowler, the many debates about the relevance of TDD, agile practices, etc, and they have offices all over the world. They started work on tools, like CruiseControl, and open sourced this. To me it never felt like they made tools open source first, but rather a need that came to have them stay relevant as other, often times better Open Source-first tools came around. The same happened with GoCD.
ThoughtBot came around later, and feels much like a adhering to a lot of the similar cultural habits.
Note: I worked for TW, currently at Red Hat.
Note 2: Somne of TW's projects under the other pillars are open source first, but aren't related to this topic.
https://github.com/thoughtbot
https://github.com/thoughtworks