My guess is that he was focused on these facts:
1) I own Wordpress
2) WPEngine is profiting from Wordpress and I'm not benefiting
3) This is unfair
And it was stuck in his mind like a thorn, irritating him whenever the thought arose, and never went away. Commercializing open source is hard for myriad reasons, but wordpress.com is actually rather profitable, and yet it still bothered him that he wasn't getting a cut.
Eventually, after many grumpy ruminations on it, the answer was obvious: "I deserve a cut of WPEngine's income, since they're using my software." No, this isn't how the license works, and there's no legal basis for it, but it felt right and fair.
This thought, irrational and deluded as it was, wedged in his psyche and fed into his deep loathing for WPEngine. All the subsequent actions follow from it, from the initial ultimatum to the various actions he's taking to fight his enemy.
This is an intensely personal and emotional fight for him, and everyone that disagrees is an enemy too. He's just asking for what's fair, and yet all these ignorant commenters on the internet can't see it.