[waltjp]

Hard to say without seeing it, but here's the definition -

A defensive player shall neither run into the kicker nor holder ... other than when:

a. Contact is unavoidable because it is not reasonably certain that a kick will be made.
b. The defense touches the kick near the kicker and contact is unavoidable.
c. Contact is slight and is partially caused by movement of the kicker.
d. Contact is caused by R being blocked into the kicker or holder by K.

Sounds like condition d. may have applied but if the defender, after being blocked, saw it as a free pass to hit the kicker it should probably be flagged. I'd also be very careful if I thought the defender deliberately threw himself at the kicker’s knee.