Yes, the propane from the grill that was connected to a cylinder that was open caused the explosion. Ignition source may have been a lit cigarette.
FWIW, years ago, I was transporting 6 20 lb propane cylinders in the back of an unmodified F150 Econoline Van. They were secured but had recently been filled by an attendant at a local fill station. Less than 5 miles from the station, the pressure relief valve on one tank opened. Within milliseconds I couldn't see my hands on the wheel. I was able to glide to a stop on the shoulder and exit the van, after turning the ignition key and locking in Park. That was about the longest 10 seconds of my life. Luckily, it was late Summer and I chose to drive with the window down.
Later, while camped, another cylinder opened because of over-pressure. We surmised that the cylinders had been over-filled.