It seems that we're getting an intermittent crash when killing a pawn that's executing a parallel node. It's crashing in BehaviourTree.cpp on this code:
UpdateInstance.ParallelTasks[ParallelTaskIdx].Status = EBTTaskStatus::Aborting;
bWaitingForAbortingTasks = true;
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-16959 in the post.