Not really a shock, I'm generally in favour of always giving guys the benefit of the doubt, for the most part until they actually fail some form of test.. an innocent until proven guilty policy if you will as there are guys that are just genetically quite blessed.
That being said, when he did the staredown with Hunt at one of those pre-UFC 200 press conferences he looked absolutely massive, looked much bigger than he did during his first UFC run. Looked about 290-300 pounds.. not that being big is a sure fire indication of foul play but still, quite a boost in size for a 39 year old considering he was reportedly walking around right on 265lbs for most of his fights during his initial run & didn't quite look as.. Hulk like IMO.
I've seen a couple of places I follow on Twitter that are MMA related say the failure is rumoured to be for some form of diuretic.. take that with a pinch of salt though as the information could have come from anywhere.