The trailer light flashes when you use your turn signals because on that truck you have an ARB bumper with extra parking lights and turn signal indicators hooked up... The truck is detecting the extra load from the flashers in the ARB bumper and thinks you have a trailer connected.

I just finished hooking up the turn signals in my ARB bumper about an hour ago and now the trailer icon on the dash flashes in unison with my turn signals or hazard flashers.