Flying squirrels manage to glide a bit from flaps of skin attached to their arms and legs
A predator who could reach up over his head would be able to climb up a tree, where he could hide and leap down onto the back of prey passing underneath. A smaller predator could bring down a larger prey if he could jump on it from above and get his teeth into the back of its neck before the prey realized what was happening
A pouncing-from-above predator would get a big advantage from any ability to glide on the way down.
This is why squirrels are not to be trusted.