Dave Meltzer noted on last night's episode of Wrestling Observer Radio (subscription required) that there are many people in WWE that are "not unhappy" that CM Punk apparently quit last week. Meltzer noted that there "was a lot of negativity towards him from a lot of people in the company."
Meltzer also stated that Vince McMahon wants Punk to return since there aren't many other top stars in the company. As for why Punk left, Meltzer said that there may not one specific reason. As to rumors about Punk leaving because Daniel Bryan was replacing him against Triple H at WrestleMania, he said that the direction was changed only after Punk quit and Punk was scheduled to face Triple H at the show until that point.
As noted earlier, Punk was not happy to be facing Triple H at WrestleMania. With his contract expiring in July and people within the company not expecting him to re-sign, he likely knew the direction that the match would go in.