The more events, the more wear and tear on the athletes' bodies. Putting on a fight card is not like putting on a broadway play. Each fight is severely taxing on the human body, in almost every case.
Look at what's happened in the WWE; professional wrestlers, despite the constant cheap shot of "fake" wrestling with worked endings, push their bodies to the limit and too many wind up dying at far too young an age. Those who survive typically endure a lifetime of severe pain.
Although the neurological effects of MMA fighting are likely a bit less severe than in boxing for example, with it's 12 round fights, and with the head and upper body being the sole targets of attack, or professional wrestling, in which athletes sometimes travel 300 days a year, and wrestle 3X a week, it's clear that a number of MMA fighters are pushing the limits of what their bodies can tolerate, also.
Nogueira and Rampage come to mind immediately. I'm a little worried about Forrest's speech patterns as well, which sound slurred and slow.