Usually excessive charging->creation of hydrogen gas->mix with atmosphere->spark or some type of ignition source->BOOM. Battery explosions can be pretty impressive. Had one on a race setup '67 Camaro years ago. Battery had gone flat between races. Push charged it. Started up for second round and KABOOM. Took out the front edge of the hood, part of the radiator support, radiator, grille on the left and headlight. Pretty powerful blast. Cleanup involves flusing with lots of water and neutralizing acid with an alkaline substance. Back then we used baking soda, but now any alkaline cleaner with sodium hydroxide in it will do the trick. Watch your eyes and clothing.
1987 BMW 325
[This message has been edited by jeffsr (edited 06-18-2000).]