2 issues:
1st base game boss heavy attack animations do not correspond with their actual "updated" timing. for instance, the twins in the dungeon, one uses a heavy attack that hits about half a second before the animation shows the swing. Since most players block at the last instance visually, myself included, we get hit by the attack because the hit happened before the swing.
2nd Mobs scatter to the winds: when a player in a group rushes in a group area like a dungeon or delve, only a few mobs actually chase that person they see, the rest go after the group members which may be far out of sight. I've seen a player get attacked by mobs as he finally loaded into a dungeon. Logically speaking, mobs should go after the target they see until they are hit by someone else, soft or hard taunted. If a player decides to 'rush' the dungeon, he should be the one with a veritable swarm of foes on him rather than the innocent group members left behind.