Battery blowing voltage regulators "why"

A short is accumulation of flaked off plate material on the bottom. It might come and go with vibration and bumps. There is also an open when the internal series connection become intermittent due to sulfation or loose crimp.

The short may max out the field, trying to raise the voltage to normal with one less cell. The open also leads to spikes in voltage and current.

Measuring a battery unloaded is not a very good test, because current is low, up the current, for starting, the open problem may show.