The MoP world bosses have invulnerable timers for the first 10 seconds they are alive. This was a change made because people were one shotting them and Blizz wanted to make sure everyone going for it could get a tag.
It’s almost as if they gave a flight ability where flying is availible. Imagine if a druid could fly in a dungeon, everyone would just play druids or kick you from the group if you don’t play one.
“whether that’s Chromie Time quests, clearing old raids for transmog, pet battling” - Blizzard
This is sound and logical reasoning. /nod