Rear Tyre torn at rim - what was the cause?
You mentioned that it's your rear wheel and that you tow a heavy load. Rear wheels in these setups get a lot of lateral (sideways) loading from the trailer. 

This can put stress on the bead that it's not designed for. A higher pressure would help stop the tire from rolling side to side -- but you've said you're already on the high end of its pressure range. Maybe look for a tire that is known to have stiff or reinforced sidewalls. 

I'm going to call herniated sidewalls: a "criggie" from now on.