A few things:
The NPC pathing was bugged at 3.3 launch, they admitted it, made it better. It caused things to run behind you constantly. That said I feel its gotten worse again but thats probably just me being lazy.
With regards to what the rogue said, hes partially right but his reasoning is wrong. In TBC and vanilla tanks had to worry about crushing blows, which NPCs 3 levels and higher than you could do to you which was a 50% critical. In WOTLK, its +4 levels, so once 80, you never have to worry about it. Thats where he was wrong.
Where he was right is that if something is hitting you in the back, you cannot avoid its attacks unless it misses. Thus having your back turned to an NPC can make you take a lot of damage quickly as they connect constantly. Hence the "pew pew" of you getting 3 shot. Also, your back turned means you arent meleeing the mob, reducing your threat.
In an AOE pack, it is inevitable something will get behind you. Backpedalling slightly will slowly move things in front of you, where you take less damage and do more threat due to being able to being able to use your offensive abilities. Backing into a wall or corner has similar effects.