You joined with dps role selected and without even so much as asking from from the rest of the group if it is ok, started to act as if you were the tank. I would kick you too and without warning. Clearly you knew what you were doing also.
Also, they may of vote kicked him simply because he was a tank in a DPS role. It annoys a lot of people. Since he didn't mention field respeccing to DPS at any point, he's half the reason mobs took so long to kill.