For the wondering if it's a bug that they aggro right before they go back to sleep, it's not. Dragons are sound aggro, yes, however there's also a blink of sight aggro before they go back into their natural sleep position and snooze away.

If you're falling down the ledge to get to Louhi's room with 20 other people and someone doesn't walk by it, it wakes up. People think it's fine that he got his victim and will keep walking up to him and that's where the trouble starts. His sight aggro is now activated and he will keep killing anyone he sees until people eventually get the hint after seeing him chew through 10+ bodies non-stop that it's best to wait at the other end of the ledge they landed on so he will go back into sound mode.