My dragon slaying strategy right now is using dragonrend to get the dragon to land and then hitting it with dual spark spells. The spark spells deal magicka damage as well as health damage. I have been noticing something interesting part way into these battles. If I'm able to keep a spark spell on the dragon for long enough, it eventually stops using it's breath attacks and settles for trying to bite my head off, typically for the rest of the battle, save a short burst or two occasionally. I don't remember getting this behavior during the mundane dragon battles (using pointy objects). This leads me to believe that in the dragons, the devs might have decided to limit the breath attacks by using magicka rather than a recharge timer like the player gets for the thu'ums.
So, are the dragon's thu'ums linked to their magicka, or is just some odd behavior?