 They're swarming to eat the meat of a dead carcass. Many times people swarm to see a preacher that is doing some great exploit and has a message of hope that attracts all kinds of people. The problem is that the meat is rotten. But if you're a vulture, you don't care. They're only interested in satisfying their desires. This depicts what sin is like. It is attractive to those that are walking in the flesh.