The slamming into an obstacle is caused by an uncontrolled stop of a Flying status.
Flying status can result from several actions. There are a few categories, so I will sort them. A "C" denotes controll over the flight, while "U" means no control, a "H" means the flight is horisontal, a "V" means vertical.
Likely in adventure mode
Flight movement type (Typicaly winged creatures) C HV
Jumping (Only player is able to do it) CU H
Falling U V
Being thrown using wrestling U H
Being slammed into U H
Being propelled by the force of a blow U H
Unlikely in adventure mode
Being knocked out and away by cave-in dust U H
Being flinged by a rising bridge U V
Now, there are several factors about the categories.
Any non-controlled horisontal flight along the ground can result in a body part skidding along the ground, which which may result (among other things) in it being severed.
Any uncontrolled flight will eventually become a fall, if not stopped by other measures.
Any uncontrolled flight may result in slamming into a creature and, if said creature is light enough, making it fly away. Slamming into a creature inflicts no damage and stops flight of the slammer.
An uncontrolled horisontal ends if the Flying creature slams into an obstacle such as a wall or a tree.
A fall ends when the falling creature slams into an obstacle (the ground), likely taking blunt damage.
A creature with at least one free [GRASP] limb in an uncontrolled flight may react by grabbing terrain or next to them or foliage above them.
Jumping on flat ground or one level below is generaly safe, but more than that may turn the jump into a fall.
Without mods, there are only a few options.
A) Creatures started on a tree and are now falling off it.
B) Something is generating cave-in dust. (A titan? An FB? Clown? Badly generated terrain? (Last one would display a "Something has collapsed on the surface" message.))
C) Someone is throwing using wrestling. (A megabeast? Semimegabeast? Rediculously overpowered Historical figure?)
D) Someone is propelling the creatures with the force of their blows. (Bronze collosus?)