My initial thought would be, as you suggest, to simply make way, but a house rule could perhaps be applied, as I do not believe such a situation is covered by the official rules?
What do you think of this:
"If a falling model would land on top of another, or several other models, this/these model(s) is/are moved, by the shortest possible distance, to make way for the falling model. This/these model(s) is/are knocked prone, and suffer a hit matching the falling model's strength".
I can see how odd situations, where the hit model(s) are unable to move because of other models or terrain, are not covered by the rule above, but it's a start
