... could be, your drivers draw more current from the +5V of the Arduino, than the voltage regulator can deliver -- so the +5V of the Arduino breaks down and the program stops.
If it's working fine without connecting, then the +5V for the drivers are sourced elsewere ... so better let it "open" ...
If it's working fine without connecting, then the +5V for the drivers are sourced elsewere ... so better let it "open" ...