Father time is catching up with him. In the Pride days he was notorious for absorbing an inhumanly amount of punishment (look at the Fedor and Crocop fights) so at 36, he's well beyond the average person his age in terms of MMA years. Anderson is a year older than him and fights like he's 10 years younger.
Also, its been 10 months since his arm was snapped. I know he couldn't train for like 5-6 months. If your body is used to working out, I can't imagine how much of a shit your metabolism takes when you have to go from 100 to 0 immediately.