wow a....interesting converation..
anyway in short, it was both. the wildebeasts were a big part of what killed him, but if he didn't fall in the first place, he wouldn't have died. The impact was hard and it happened to be on the animals. if someone fell out of a plane and hit a rock would it be the fall or the rock? and the answer would be both because it was the force of the fall that would cause the rock to be deadly.
now for more detail.
for one, this is a movie, a cartoon movie, so they don't have to be super realistic, he could have just as easily died from falling. and the fact that Mufasa is a cat doesn't mean he would have landed ok. I know of real cats that have died landing wrong. And I've seen my own two cats miss landings before.
First, he was likely already injured before he got up to Scar, we see him getting hit by the wildebeasts before he makes the big leap. He was already exhausted and you could tell that in his plea to Scar.
Now, Scar was his brother. Mufasa was completely in shock when Scar pushed him off the cliff, therefore he wasn't likely to react with quick thinking to try to save his life. Plus, again he was drained.
He fell backwards, onto the herd, probably hitting several of them. horns have killed lions before. so he was either dead at this point or very hurt.
he then eventually fell to the ground, (and who knows how exactly he landed), and thus was trampled by probably a good number of the animals leaving him most certainly dead.
in the end, he would have been dead either way, because that's the plot, and Scar never would have stopped trying anyway. Disney didn't have to have the wildebeast there to kill him, but it adds to the drama of the scene.