That could be the problem. GSB for Windows (i.e., the mainline version of the game) is now at 1.62 – the Mac version ported over to OS X by Red Marble Games seems to be very far behind! I know a new AI behaviour was added a few patches ago. Do you have the option for the combat order called “Last Stand”?
EDIT: That’s not it… Movement was redone in v1.61 though. Could that be related?
It was added back in v1.54 on Windows, which was several months ago at least. I think it may have even been pre-Parasites, pre-Outcasts for sure. You sure you’ve got the latest Mac patch?