Negative Battery Cable Fire

What is your question?

This is on a 2004 National Ragen 5th Wheel Trailer. I recently had the two 12volt batteries removed and I had 2 6volt bateries installed. I took the trailer home and a few months later I turned the eletrical kill switch on for electrical power and the negative battery cable caught on fire completely burning the cable and the two new 6 volt batteries. Does anyone have a reason for this to happen?