Probably, the founders believed that the toxicity of the internal culture had reached a point where it was extremely detrimental to the workplace. And, knowing DHH, it was probably not only a financial decision but a lifestyle one. He may have realized that the advocates that actively discussed and promoted political issues within the org was making his life and others in the company miserable.
Would you allow a Jehovah's Witness to be a significant part of your life if they proselytize you every day? I definitely wouldn't. Some of these activists probably think that's their right to do so; so let them walk.
I'd be really surprised if this surprised the executive team. This seems to be a super intentional and very professionally executed cultural shift / purge. Professional, because you give them freaking 6 months of severance and you don't call them out or force them out. If that isn't recognized as goodwill, you're blinded by your political disagreements to seeing how nice of a treatment that is.
The people leaving probably grew with the company, and contributed a lot to the company, so giving them 6 months of severance is recognizing that. But it's also telling them, hey we are making a cultural shift in our identity, and if that identity is no longer you, sorry, we'll treat you really well as you find a place that supports your identity.
This reeks of professionalism and GOOD management to me.