What we believe is that if you aren't initially near the boss when it gets pulled you don't get the "Drouk Hunter" buff. If you attempt to engage Dreadtooth without "Drouk Hunter" you are going to be 1shot. This is of course to prevent people from helping a group fight the boss.
This is what I we were thinking as well.
Thing to note, I believe if you are stealthed and pull, you will get the "Dread" debuff as well. Can't confirm it, but when we did some testing on it both myself (Operative) and our tank (Assassin) were within 10m from the boss, stealthed when pulled, and we got the debuff and were killed.