i2c and usb comm interference

I'm sending some commands from a phone to an arduino Mega ADK via USB and then using i2c to relay those commands from the arduino to some motors. Individually, both parts work. I can manipulate LEDs from the phone as desired or I can write a sketch to cycle the motors independently.

I am running into problems, though, when I combine them. The arduino is no longer receiving the usb commands correctly. If I take the app that controls the LEDs from the phone, but plug the i2c cable into the arduino, it suddenly stops working. The arduino is powered from an external battery so power should not be the issue.

Is there some interference that goes on between these two protocols?