Nobody knows for sure whether it was his WM position that was the biggest impact on him leaving. I personally think it was his injuries mixed with his thinking that he wasn't in a position in the company he thinks he deserves. If he knew he was about to lose at Wrestlemania to HHH -- which is very probable -- then I understand why he probably doesn't want to do that. Some people seem to think it's because he wanted to be in the main event, I highly doubt it. A match with HHH at Wrestlemania is pretty close to the main event and since Punk was pushing for Bryan to get pushed, then it's very unlikely that he thought he deserves Bryan's spot.
It's a combination between injuries and not wanting to job to HHH at Wrestlemania.
Leaving due to injuries is fine by me. I'm not sure about him doing the job though, I wasn't keeping up with reports/trusting them a lot, so I have to say that I'm not particularly sure he knew he was going to job, but assuming that was the case, he has a bit more of a reason here to leave but I still think it was overreacting, so to speak. But if Punk had a problem with losing to Hunter, how is it that he doesn't want to be in the main event? You said it yourself that Punk/HHH would be ME caliber, so I'm not sure what he wanted his position to be (assuming he was OK with not winning the belt). What I'm getting at here is that if he was OK with DB getting the push I don't know what he could possibly be angry about, because he'd be at the second most important match on the card. In that case the guy was leaving to avoid doing a job, which, again, is kinda childish. Granted, we don't know if he left mostly because of injuries or what it was, but if he was going to stay and left because he was doing a job... you know. I get standing up for himself or not taking s***, but losing a match when you're still in the ME is not something that gives you the right to leave as if you were being mistreated in my opinion.