I've not heard of or seen the condition you describe. Did you take any pictures of the situation that you could post here? Its always helpful to actually see the issue and then to try to analyze these problems. Here in Florida, we rarely grow Haas.....perhaps some folks out west have heard of or experienced this problem.

Harry

Either squirrels or-,most likely at that size- the tree aborted the small fruits. Avocado's from my experience wont hold fruit until the are at least 6-8' tall. Grafting makes them bloom very young..and every single flower will fall off and any fruit that do get to some size also.
The tree's seem to know that they need to have some branch diameter and roots and enough foliage to support the heavy fruits.

Sorry Harry, I have no pictures

and Thanks stanofh, that's a very logical explanation!