Recently I installed HybridReverb2 to experiment with convolution reverb. However, I found that it causes AM to crash on exit with any patch where it's instantiated. (This is on OS X 10.6.8, AM 2.2.3). Uninstalling it solved the problem. I'd read on the KVR forum that it's buggy, which seems to be the case:
Thanks, Ross, for helping me locate the gremlin in the machine.