He went to europe while they were in the middle of crunch time, then promptly took a week of sick time upon his return. They fired him shortly thereafter. Steve talks about it here:
In Aaron's words: "Yeah. I was unhappy working in an office and didn’t hide it. So I’d come in late and set up lots of off-site meetings and stuff. And my boss wasn’t really thrilled about that. Also, I think he was upset about me disappearing for so long on vacation. One of the places I went to in Europe was the Chaos Computer Conference. And while I was there I hung out with my friend Quinn Norton, who was reporting on the event for Wired. She took my photo for one of her articles and it was featured on wired.com’s front page. “Heh,” I joked. “I bet the first time my boss finds out where I am is when he sees my photo on the front page of his own website.”"
So yeah, he earned his firing and the emnity of his coworkers the old-fashioned way.
Edit: Also, there's some speculation on the thread I linked to that Aaron was a cofounder who received a payout in the CN deal. He wrote web.py, sure, but if he walked away with $1MM+ without showing up to work, that could explain why they still hate his guts.
I know nothing of the internal arrangements and disputes. But knowing how these things often go, it's easy to imagine that Aaron was tangentially involved, and the Reddit team needed more help, but Aaron still had another project, where his 'founding' stake/claim was larger.
Offering a founder-ish stake and founder-ish title for bringing that other project in-house (even if the combined team pursues something else) is a common resolution to such a situation.