Firstly...very nice design.
Secondly...just like when a fighter gets a ring fighting name....your robot should be called scorpion...because thats what the PEOPLE will call it
On to serious business...
I noticed that your robot has a point when you accelerate when the high COG (center of gravity) causes you to pop back. If you accelerate too quickly you might run the risk of falling over.
One way to combat this is to prevent very hard accelerations with you motors. You can do this by implementing a "ramping" algorithm into your motor code.
Basically, you program a shape to a ramp (either linear in the form of
motor speed = joystick_value *(x)
where x changes from zero percent to 100 percent over a set time period
You can also do sinusoidal ramps
motor speed = joystick_value * cos(theta)
where theta goes from 90 to 0. This will give you more of a ramping curve.
Hope this helps you some!