Floor rot is always caused by moisture. It is most common around the shell because almost any exterior leak will run down the inside walls and come out at the floor. This can be from a bad window seal, vent gasket, failing Vulkem, a popped rivet, or any number of sources. However, in bathrooms you always have to consider the area around the toilet. In my own case I had some rot around the opening where the galley sink drain went into the grey tank - some 18" or more away from the shell.
The moral is to be vigilant about leaks, and get them fixed.