Yeah what is this?!!??
I see jumping in two cases usually:
1) Someone jumping up and down trying to break my rifle's lock on them. Like some 'raff did last night consistently... right before he would die from my rifle gunning him down.
2) People bouncing up and down due to some wierd lag or something. Mule was doing this last night as I was attempting to capture a zone and we were both on the ground. There is no way to see where the bouncers really are so you just run around wildly tossing grenades and swinging your knife.