On our first trailer we had a soft floor develop. How it happened.... goes back to a week after we got it. I noticed one of the tires didn't match the others. It was actually a lower weight rating. And the floor that rotted was directly behind that wheel.
My guess is that the delivery driver blew out a tire on the trip from the factory. That tire tore up the fabric that kept water from shooting up onto the underside of the wood floor when towing in the rain. 4 years later I was replacing floor.
So after that long story... Check under the trailer for problems also.