solar panel blowout

I have an 80 watt solar panel installed on top of my 5th wheel. It was originally a portable solar panel that was put permanently on top. The controller was left attached to the underside of the panel. It was connected to two 6 volt marine batteries in the 5th wheel. I have a kill switch so there was no power going into the trailer. Trailer was unused for about 5 months. Went to check it out and found that the cord from the solar panel to the batteries was fried at the fuse and in two separate pieces. Batteries were dead. Any suggestions on why this happened.