As a Tank in FFXI, WoW and now also ARR I can tell you several reasons why a Tank might appear like a d-bag.

Something goes wrong leading to a wipe, its the tanks fault.
The Ranger/Hunter/Archer dies, its the tanks fault.
The BLM/Warlock/any DPS Magic Class in ARR dies, its the tanks faul.
Someone runs somewhere alone gets beaten to a pulp in a second, its the tanks fault.
Someone pulls a group of enemies and enganges without the tank to produce a BIT of hate and dies, its the tanks fault.
Someone dies because the healer is AFK, its the tanks fault.

So no matter why and who messed up, a lot of people point at the tank. Now try and stay friendly throught all those times. Why tanks act like they are the boss, lets face it, most of the time they are with the healer on same level of command. You don't like the attitude of Tanks you play with? Become a Tank yourself, you don't want to? Now WHY is that?