what'd i do wrong?

[Leknaat]

Medica is a hate magnet
Overcuring is just as bad. The whm might also have been throwing regen on you which will keep ticking and generating hate even if it's not actually healing you. Early levels some clueless healers will even toss regen on you before a pull (worst idea ever) overcuring hate to instantly rip it off the tank.

One of the better things you can pick up as a tank is waiting for stoneskin (this is in the 30s for a whm/cnj) and/or whatever the scholar one is before a big pull, and not pulling while regen is still ticking from a previous pull. The added damage you can soak before the healer has to start casting on you will help cement the mobs a bit better to you.