Hmm, excessive gas flow, no doubt...
Possible causes would be a defective gas regulator which could be sticking with the internal valve at or near full open. Basically the the regulator valve can't back down the gas flow to run at normal pressure to the burner control valve, and this will cause a huge amount of propane to flow through the valve, even when set on low, as it has too much pressure going into the control valve.
I would suspect that the cause could very well be some dirt or other debris which may have gotten into the inlet of the regulator while uncoupled from the tank at some point in time. Then, it could be forced into the valve seat when coupled to the tank upon opening of the tank valve from the rush of gas from the tank which would occur at that point.
I'd say you better be hunting for a new low pressure regulator. If you can't get one from the manufacturer, try your local hardware store. I had a ruptured regulator diaphram on my small GOSM about 1-1/2 years ago, and found an acceptable replacement that way. It was a Mr Heater brand, likely for a forced air propane space heater, but it worked out fine to retrofit to my GOSM. You may need to bush up from the 1/4" inlet size of the regulator hose to a 3/8" size of the replacement regulator which is what I had to do, but the pressure rating is the same as what I had with the original, which is the most important.
Most replacement grill regulators will more than handle the small 13,500 BTU rating of the small GOSM burner, as grills are inb the 30K-50K range, and some are even higher.
Sux, man...been there...mid-smoke and find a SNAFU like that. At least you had a back-up smoker, so hopefully the bird will survive from the soot exposure.
Hope you didn't loose too much valuable time and internal heat of the bird so you can get through the danger zone OK.