I think that in most (if not all) states it is illegal to change lanes within a certain yardage before/after an intersection. For instance 100 yards before/50 yards after, etc. That probobly coupled with your weak signalling is probobly why you got popped. I know that depending on where you are in the country that law gets enforced differently, but I do know it exists.
It's there to save people that might be turning right on red or pedestrians from getting hit because a car wasn't following their existing course as they entered the intersection.