Most people don't realize that it isn't just the four months time frame between the KO, the fight was announced in November so that means he was already either training or revving up for a camp. He never had even two months to really recover, because he was already in camp sparring and taking blows to his head in January at the latest. (via his YouTube channel)
Putting your body through that training, weight cut, sparring, while not getting two months to recuperate is crazy. A lot of times when fighters are very successful, nobody wants to step up and offer their opinion, you always need someone who will protect you from yourself.
That was the first time he'd been KO'd on the world stage; he was probably dealing with a ton of issues behind the scenes and figured he'd go in there and get a W like he had so many times before. GSP is a good example of a fighter who got his butt kicked a couple times and started dialing in on better decision making and preparation, but he had the fortune of losing when he was young. When you go basically your whole career without losing, your decision making, and preparation start to fall off.
When you're a champion you have to take the necessary steps to be at your best, because once you lose your title you aren't making any real money. Assuming Volk is on a $1 per PPV deal, he lost roughly 30-40% of his damn pay. You don't want that stuff on your conscious, because the doubt regarding his recovery period is going to be on his mind for a long while.