Oops! The simple answer is, the heap is getting corrupted (always bad ;-) and the OS Chooser extension is running into it and dying.
This now uncommon problem seems to result from various combinations of extensions conflicting with the OS Chooser. You can try to isolate the problem if you have Conflict Catcher or Now Startup Manager.
Otherwise, disable the OS Chooser extension, and instead launch the BeOS by using the "BeOS Launcher" utility, located in the "BeOS Mac Tools" folder.