Why do planes explode when they crash?

Most of them don't. If you look at the NTSB (or AAIB for UK) then you will see that a very small percentage of crashes result in fires. Even less in explosions. read more

In this case aircraft will always explode on impact, often even before impact unless of course the hero needs to escape in which case they will explode at the point where the camera can get a great shot of flames over the shoulder of the hero. read more

The reason they"explode" on crashing in the movies or on TV is the same reason cars always explode as they sail through the air after running off a cliff-side road - the FX people installed a"bomb" in the car (or airplane) and triggered it by a radio signal to make a dramatic effect. read more