There's gravity in space. There's gravity everywhere. Destroyed ships should fall or drift towards the largest object near them.
The ships also probably fall suddenly because their anti gravity generators have been destroyed, and they're heckin heavy.
Explosions also tend to change the velocity of the things around it and in Star Wars they always attack the tops of ships meaning the explosions will most likely be on the top of the ship. When the grav generators go out the explosions will keep pushing it down too
The ships also probably fall suddenly because their anti gravity generators have been destroyed, and they're heckin heavy.