It could be a wheel bearing out of adjustment (loose) that allows the inside edge of the tire to contact the road more than the rest of the tire.
It could also be a tire that has a belt separating, and should be replaced.
It could be that axle is out of alignment, and you need the wheels aligned.
Has anything else changed? New tow vehicle, putting a motorcycle in the bed of the tow vehicle, a keg of beer in the front left corner of the trailer...
Meddle not in the affairs of dragons, for you are crunchy, and taste good with ketchup.