Well, have you ever been camping in bear territory? Essentially you have to hide all your food and food-like items in order to make sure the hungry bear community doesn’t tear through your tent/other gear like a tornado seeking the one candy bar you left around. So most people hang their stuff high from a tree branch or the like.
Every so often a bear will get really, really hungry and will actually climb the tree and skinny out on a branch to get at the grub. Most of the time the branch breaks first, dumping both bear and food into a furiously-rummaging-and-eating pile on the ground below. But every so often – especially if you pick a really sturdy branch – the bear makes it all the way out only to tumble off and get tangled up in the cord mid-fall. Sadly, this generally means the bear ends up hung like a bread thief in '60s Turkey.
Hence, “hung like a bear.”