Yellow back sliders behaviour

Question
I have two yellow back sliders they are about one year old.  One is bigger than the other the problem i have is that the bigger turtle bites (Or looks like it is) the little turtles shell between its back legs and starts shaking it.  It will only stop when i tap on the tank.  Do you know why the big turtle is doing this, it is starting to worry me as i don't want the little one getting hurt.  They do get along together the little turtle climbs on the bigger ones shell and the bask together under the UV light.  The tank is keep in very good condition it is clean i have a pump, filter, heater and UV light heat is kept between 28-30 degrees. I hope you can give me some advice.

Answer
These are 'Yellow-BELLIED' Sliders- the right name will make it easier to research.

This sounds like domination on the part of the bigger turtle. This is common when they are crowded, or if both are males, etc.

I don't really do water turtle questions in depth. If you would like more info, try http://www.turtleforum.com