What is it called when you push someone over another person?!

I say someone the other day trip someone in a way that I haven't seen since high school!. First, one person crouches behind someone who is standing straight up without them knowing!. A third person approaches the one standing seemingly to make a conversation but precedes to push this person backwards over the guy on the ground!. The result is the victim falling flat on his or her back in a hilarious manner!.

Does anyone know what this is called!? I am trying to find some funny videos but don't know how to search for it!. Thanks!Www@Enter-QA@Com

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