I think this is really normal! And yes, sometimes can be excitement and the squeals etc from the other child can reinforce it as an exciting thing to do.

I have stopped this in my toddlers through 'no, don't push, you'll hurt [insert name] look she's crying, give her a huggle' etc and I have also used timeout for repeated pushing after I've asked not to etc... I had planned not to use timeout but I have 4 kids, incl until recently, 3 under 3, and I just found I had to use it to get things a bit calmer.

Mine have responded well to pretty much what you are saying, I think you can't expect overnight success perhaps? but if you keep giving this message that it's not a behaviour that we like and that it hurts and show them a better behaviour then you will get there.