A couple of other people have experienced this issue in the past with Unity, where the Unity segmentation sample runs fine but using Enable3DSeg in their own application causes Unity to crash. An example of this:
In another example of the problem, an SR300 user was able to use segmentation and hand gesture features successfully individually, but if he tried using the the two features together at the same time then their program wouldn't work. His suspicion was that the problem was with the SenseManager.
So far, nobody has managed to solve the issue yet. Reading back through the posting of the first example though, I note that they were having the problem when calling SenseManager.Enable3DSeg().