too much gas in the empty part of the canister, really, not enough air

most of the time, even before you shake it, a gas can will not explode even if you put a lighter right up to the open mouth, but there's a good chance once it starts heating up the air it can warm the gas and that will lead to a massive amount of boiling gas to pump out the nozzle and actually explode

i once set off a small explosion involving naptha (zippo fluid)... there was probably less than an ounce of gas in the pot but it was all boiling, and then i bumped the pot against the resistive elements and with all that gas spilling over the edge it caught fire and the whole ceiling was turned black and i had 2nd degree burn on my arm where it went off

not saying "don't be careful" but rather, retards will get away with this, until they don't