Sometimes there are high winds, or some insects eat the tree so that it dies and possibly rots, causing it to fall.
"Natural" forests that are not for economical uses have lot of fallen trees, rotting logs and stuff like that in them. Forests in HnH are very pristine with only very few old logs here and there.
Trees grow back these days, so I think that trees should also occasionally fall by themselves, then slowly turn to old logs and old stumps (already happens) and then those decay away (maybe doesn't happen yet)
