I still wonder how all these corrupt catalog problems occur

I wonder if there is a bug in the program (and if it is the case, what cause that, since I really do not see what it could be), or if it is due to another program or some hardware/software problem.
As the .bak file is corrupted too, there is really little chance that it is due to AMC: when saving the current file is renamed to .bak and a new .amc is created. So the .bak was a valid loaded catalog, and not modified since then.