Aluminum electrolytics explode. After all, they're pretty much just a steel can with some aluminum foil and salty, acidic water in them. Put them in backwards and they overheat. They overheat and the water boils, expands, and explodes the metal can. How long it takes to blow up depends on the reverse (or over) voltage applied, as well as the thermal mass of the part itself; bigger caps take longer to blow just like bigger pots take longer to boil.