Apparently the collar was not centered in the opening and the bladder material found its way out around the collar. I suspect the hot sun increased the pressure on the tubes but since the bladder was not contained by the cover, it kept expanding until it burst.
I urge you to be sure the valve is centered whenever you inflate but especially if the dinghy will be unattended for any time and you might not notice a problem. Since this is the first time we have encountered this it's probably an outlier but an ounce of prevention....
Our new Dinghy Dogs design has an improved valve arrangement making this problem nearly impossible as seen here:
Wishing us all a great New Year and sailing season.