I think most of the reason there's been a lot of tipping the last couple years (bear with me I'm a sophomore..clueless on before that) is because of the uneven playing field. When you're higher than another part..you can always fall over and then be laying on your back...same on a ramp, if you're pushing another bot, and they happen to have sloped sides...up..and back.
There's always those things about high CG... driving... how you move an arm... such things that can make the robot tip over.
Best to just be careful and hope for good luck

Ellen