The different healing powers have different enmity mods. Regen has an enmity modifier of 1. Cure has an enmity modifier of .5. Cure II and a lot of the bigger heals have an enmity modifier of .8.
Barring Provoke on a target that you have absolutely no threat on while someone else has a crapton, Medica II is the highest enmity generating power *in the game*: it'll hit 8 targets for a potency 200 heal at a .8 modifier and then tick for 500 more over 15 seconds at a modifier of 1. That's 5280 enmity as spell potency *from a single 3 second cast*.
No healer should cast Regen or Medica II *any time* close to an add spawning: 10 seconds before and after a new enemy enters combat is Medica II/Regen no-time. If a healer casts either of those spells during that time, that healer better expect to take more than a few hits to the face by ripping aggro off of the tank, regardless of how good the tank is at generating aggro. Even Shroud of Saints isn't going to do enough a lot of the time: they simply generate *too much enmity* for a tank to rip back.
Most of the time that a healer rips aggro from a tank, it's not the tank's fault: healing generates aggro *really* efficiently, especially when it's AoE healing. A healer that heals indiscriminately is going to give even an *amazing* tank a helluva time.