Neuro, Considering all the recent clinical setbacks for Cortex, a more logical conclusion is that with CX-717 in serious trouble, and the high impacts soon to be outlicensed, Mansbach just doesn't see his future role at Cortex being all that attractive. He may not care much for the orphan in-licensing idea either. These would seem like the most obvious reasons, though I guess we won't know without talking to him personally. As you said, he may also figure Varney is destined to be Stoll's successor. Assuming he gets to keep his existing Cortex options, Mansbach can still participate in any future upside, while making a lot more money somewhere else, and with more interesting programs to work on.
It'll be interesting to see if anyone else decides to bail out. Varney could probably land a really great job somewhere else, but it's still too early to conclude that an "abandon ship" mentality has taken over at Cortex.