Battery blowing voltage regulators "why"

Ok guys
answer me this. I've been blowing regulators for a while. Amp meter would jump wildly. It would burn up the electronic modules and other things. Finally just for the heck of it I changed out the battery with a smaller cca battery I fhad in the shop. Problem solved.
I just want to know what could be wrong with a battery to cause this? The battery always started the motor it never need charging and when ever I checked the voltage it would be 12.50 or better. I took it to auto zone where I bought it and had it checked. The clerk said the battery showed bad, but he couldn't say what was bad just that his machine said it was bad.
Any ideas?

big block