Have you checked your running lights before? Has this happened before or is it new?
Are the hazard lights or your turn signal activated in the tow vehicle when this happens? How about the headlights? If so you may have a wiring problem.
If the hazard lights are off in the tow vehicle, do the lights blink in a repeatable fashion (like your turn signals do) or are they just flickering randomly? If they are flickering randomly it might be a loose connection (maybe where the trailer connects to the tow vehicle).
You might try shutting off the car. Disconnecting the trailer plug from your car, and then liberally spraying some contact cleaner on both sets of connections. Then plug and unplug the connector several times, make sure the plug is firmly seated in the socket, and then turn on the car and see if the problem has gone away.