I forget when this sort of thing last happened, I haven't played OFP in quite awhile, something armored didn't fire on a MBt or abrams. It might be what you're talking about. The lighter armor would be able to destroy the abrams, but I think it may have to with threat levels or something. Like M113s would run from BMPs or something....
And to make it fire, maybe use dotarget then a fire command on the Shilka.
And MFCTI the reason might be that when ordered, AI (I'm assuming AI was in control, if human I would try and put as many rounds as possible into the abrams as fast as possible) would attack because their commander ordered them to.