Rear Tyre torn at rim - what was the cause? I had a blowout the other day.  On investigation, it was a C-shaped cut in the tube.  It patched okay and on reinstalling I found this culprit :





The tyre is not new but its not old either - plenty of tread left.

I've seen very old / weathered tyres where the rubber has deteriorated and the bead has separated but the cords remain.   However this has torn about 20mm worth of cords.

I've wondered if it was too-high pressure, but the tyre sidewall says 40-65 PSI and I run them at 50-60.

The tyre may be too wide for the rim - but its the original tyre which came on this bike from new.

There is no noticeable roughness on the rim at any point, so abrasion seems unlikely.

The bike is a 20" folder and has done 1,300 km since I bought it used.  Previous owner did minimal riding, so its done maybe 1,500 km total (1000 miles)

Lastly, I do occasionally use this bike as a tractor to tow a trailer.  Its done 50km towing some amount of weight on a rear-axle mounted trailer, and despite moving some decent loads the nose-weight of the trailer has never been more than 10-12 kg (20-25 libs)

Question What was the cause of this tear in my tyre?

Edits


Yes this is the rear tyre.
Its a folding bike that started as 1x6 and this wheel was rebuilt onto an 8 speed cassette hub so its 3x8 gearing.
yes it has rim V brakes
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.