All together now:

Learn to love your graph editor!

Double click your offending action's dial, where it says 'graph' click that and have a shufti at what's going on. You'll find that the curve goes way beyond where you want it either before or after your key-frame. Just as Bobasaur says it's the interpolation thingy again. The graphs allow you to see these in action. Over on RDNA I've put some tips and tricks up that cover this and tell you an easy way to prevent it happening again.
My clean site here also has some info on this. Head for the tutorial section.