all power is in one line on a breadboard going back to the 5v pin.If that is not causing the present problem then it will cause others in the future. The Arduino voltage regulator is simply not up to the job of powering 2 servos used as drive motors. At the best the servos will move erratically and the Arduino may reset as the servos draw current and cause the 5V rail voltage to drop. This is exacerbated if the power source is not up to the job, such as using a PP3 9V battery. At worst the current drawn by the servos will burn out the Arduino power regulator.
Use an external power source for the servos such as 4 AA batteries with a common ground.