Arduino resets itself
When powered through USB there is a resettable polymeric ("poly") fuse. If you draw more than about 500mA this fuse will cut out and isolate the power.  When you remove the load the fuse will reset and allow power back to the board.

"Poly" fuses are in fact "Positive Temperature Coefficient" (PTC) thermistors. As they get hot their resistance increases (as opposed to the NTC thermistors you use for temperature sensing where the resistance decreases as they get hotter).  The current flowing through the PTC causes it to warm up. Too much current and it gets too hot, and the resistance gets too high (it's non-linear, so the resistance suddenly increases a lot).