While it’s possible he suddenly woke up one day and decided to become greedy and petty, it seems more likely that with the end of zero-percent interest rates, his investors are nagging him to increase his rate of growth of profit accordingly. Otherwise why rock the boat when, as you note, he’s already making money hand over fist? But it’s not rate of return annually that matters to investors, it’s the rate of growth of rate of return that matters, and that’s almost certainly trending negative by now.
However, if Automattic is indeed flush with unlimited runway, then the lawsuit is not in contention for any resources needed by the Wordpress project, which lends credence to this being simple market manipulation — i.e. “we have ceased Wordpress development to drive the community and userbase into pressuring WPE to increase investment into Wordpress open source work”, which only works when there are few (or no) serious competitors. It could be a precursor to taking future development of the Wordpress project closed-source, too. We can only guess.