The underlying fault was an unrecoverable memory error in the memory pool for the GPU driver while executing the WindowServer process. That means it may have been something to do with UE4, but there’s also the possibility that it is unrelated. I’ve added all the details to our internal database as part of UE-6539 for further investigation. Yosemite (10.10) has a number of important OpenGL improvements that will improve performance in UE4 so if possible we recommend that you upgrade, even if it doesn’t directly address this issue.