"How, exactly, is a bear going to escape with a pack that's hung from a tree well over its head?"
Are you thinking that hanging food is 100% effective?
There is a reason the bear cannister exists and is required in many areas.
A determined bear, will eventually get food hung from a tree, given enough time.
They can gnaw the limb off, gnaw thru the cord, drop onto the bag and pop the cord or rip it open.
They are incredibly strong acrobatic climbers. Small bears live in trees, Ive watched them eating in the top of a tree 50' off the ground. They can climb and hold on for hours on a branch with legs and one paw, using the other paw and teeth to gnaw thru something.