I think it has to do with longer levers (i.e. bones) requiring more strength to accelerate. In growth spurts, the bones grow faster than the accompanying strength increases. So, until strength catches back up, velocity drops. Seems to make sense.
House told us that in some country (don't remember which one), young athletes who grow "x" amount in "y" amount of time are removed from competetion for some period of time to allow them to play catch-up. I suppose one could be at an increased chance for injury in such a state. It's an interesting concept.