This sounds like a latency issue. Essentially, there is a latency period (0.3 seconds between when the server checks your position) in most maps and instances (HM and specific endgame dungeons have this latency reduced to 0.01 seconds or so). The result is that the game will believe you are still moving because your coordinates change while midcast. The game generally views this as you trying to move while casting and cancels your cast.

It can be a big problem and I have had a few casts where this system issue has interrupted as well. I've also had instances where I use it to my advantage to move prematurely at the end of a cast and reposition an aoe heal/holy. It just takes some getting used to.