Battery changing polarity?

Anyone ever heard of a battery changing polarity? We took the battery out of my girls Jimmy this spring to store it for the summer while she drives her car. Well I went to put it back in the other day, didnt think anything of it, hooked it back up and after about 4-5 seconds the horn just starts wailing and smoke is pouring out all over. So I rush to unhook the battery (which promptly burnt my fingers) But it was too late. The BCM is toast! So I'm standing there confused as hell. Checked to make sure I had the terminals on right, and there is no way I couldn't have cuz the opposite wire won't even reach to the other. So just for the hell of it I slapped a meter on it. It read "-12.17V"!!! I was like WTF?? So I flipped the probes around the WRONG way and it reads "12.17V" positive!!!

So now I gotta install a new BCM tomorrow. First off I'm baffled as to how the battery could switch polarity like that, the plates are oriented inside so as that's seemingly impossible. And second you would think the truck would have some sort of protection against somone hooking up a normal battery backwards somehow.

Oh well!