When batteries are charging they generate hydrogen gas.
If the battery is full there is little room for the gas to collect.
The lower the water/acid level the more room there is for the
hydrogen to collect and the bigger the BOOM!. Any gas coming
from the battery through the cap vent holes or if the caps are
off that finds an ignition source will cause it to explode. Before
putting a battery on a charger its important to first check the
water level. Nobody does.
I blew one up on my 55 Ford when I was a kid.
It was many years later when I learned why.
A "Boathouse Repair" is one thats done without having tools or the skills to do it properly.