This is addressed inA recent tweet from the head of the Star Wars Story Group, Pablo Hidalgo would suggest that (artificial) gravity plays its part in getting the bombs out of the magazine.
There's gravity on the bomber. We saw a person fall and a remote control fall. That's really all you need to get the bombs to fall out the bottom and then keep falling.
The Last Jedi: The Visual Dictionary. The indicates however that the bombs are repulsed from the rack by electro-magnetism (like a rail gun) and then drawn to their metal target by magnetic attraction.
Bombs don't technically "drop" in microgravity, but are impelled from their racks by sequenced electromagnetic plates in the clip. The bombs are then drawn magnetically to their unfortunate targets.
We can square this circle by imagining that the lowest bombs (which only fall a few metres in gravity) are given an additional boost by the magnets, in order to get them moving fast enough that the topmost bombs (which fall 20+ meters) don't end up ploughing into them.
