Comment 2 for bug 557547

This is an intentional change. Per :

"Edge flipping on move is disabled because (1) it can be triggered accidentally and confuse users, (2) it is a bit awkward to use, and (3) it's not symmetrical (it's disabled on the top edge because it is too easy to perform accidentally there)."

You can change this by installing compizconfig-settings-manager and enabling this feature again.