Ground loops can be bad. Do follow up as suggested in the comments regarding ensuring the power supply grounds are the same. To be specific, connecting the power supply grounds through the Arduino is a bad idea! The trace size should not be assumed adequate for the unknown amount of current passing between the ground outputs of the power supplies. And a voltage potential can develop causing the processor to behave unexpectedly or worse burn out components.
Also, consider that some Arduino platforms regulate power to 5 volts on the board and that many servos run fine at 5 volts. If this is true here, you could greatly simplify and possibly solve the describe problem by using a single 5 volt power supply.
Lastly, ensure there is a pull up on the 2 inputs connected to the (assumed) normally-open-momentary-contact switches. There are none in the diagram. So it is assumed pull ups have been activated in the code such as explained here.