First, how often do big names leave? Nobody would care much if, say, Curt Hawkins' contract ran out and he just "disappeared", and that's probably how WWE would play it and nobody would even realized he was gone until long after he was. But someone that moves the needle like Dean Ambrose? People are going to realize fairly quickly that he hasn't been around.
Second, word got out much earlier today before WWE addressed it that he wasn't resigning his contract and he'd be leaving after Mania. It was all over the internet before WWE addresssed it. Third, to go along with those rumors was word from an unnamed wrestler (pretty much universally accepted to be Seth Rollins) that Dean's "tired of the hokey ****". Dean wanted to have a serious heel run after he returned from injury, and instead it's been gas masks and ass injections, and supposedly it was the straw that broke the camel's back. Then there's the fact that he lost clean to Seth Rollins last night, and then proceeded to get run down by Nia Jax.
I don't disagree that this could be an elaborate work - it's the kind of thing that WWE would do. But there's the fact that Dean almost died, which will always give things a new perspective for someone. Add on top of that one of his best friends is struggling with cancer. Then top that with WWE is having him do storylines he has absolutely no interest in doing. Put yourself in this position - would you stick around?