This has become a trend after the surge of WoW players. Additionally this was solidified behaviour after tanks have become essentially obsolete with the removal of enmity generation aspected attacks.

Tanks are redundant now. It takes nothing to be a tank anymore hence why a DPS can assume this position. In fact with a RDM you can easily solo a dungeon now. Healers are also becoming obsolete with self-sustain options on tanks. (Warriors can even solo content on their own).