[for a stack of something] to crumple and fall down. The chimney toppled down in the earthquake. The woodpile toppled down during the night and scared us all to death.

to fall off the top of something very tall. (Of is usually retained before pronouns.) Careful there! You might topple off of that wall. I didn't get too close to the edge, because I was afraid of toppling off. She toppled off the wall. The vase toppled from its shelf in the quake.

[for something very tall] to fall over. I was afraid that Jimmy's stack of blocks would topple over. The stack of books toppled over and ended up as a jumbled mess on the floor.

to cause a stack of something to crumple and fall down. The earthquake toppled the chimney down. The hurricane toppled down the weakest buildings.