After this happens, what is the dome temp? Does the charcoal have flames or is it red hot yet? When you put the daisy wheel on, do you also close the draft door?
It sounds more like an oxygen issue than "gases". You have a oxygen hungry fire going then you put the cap on it restricting the exhaust. Temp continues to rise, exhaust gases are snuffing the fire, then they clear out the top after a few seconds and the O2 hits the heat......BOOM! The only place it has to go is out the bottom vent since the top is restricted.
It could be some combo of the dreaded VOC's, but while they do have "volatile" in the name, they are not explosive nor that "volatile". They off gas and burn a little, but they don't produce this kind of reaction. This just sounds like you got a fuel/air mixture in a funk and that's the reaction.
Always limit your intake before your exhaust, and problem solved.
The firegrate has no effect on cooking durations or charcoal consumption if the cooking grate temp is kept the same. You could take it out altogether and maintain 24+ hrs at 225......the amount of air coming in and the amount of smoke leaving is the same.
The only thing that the new firegrates do is eliminate a possible choke point on long low & slows.....Where the ash builds up on top of the stock plate and doesn't fall through. They do allow a lot more O2 get to the charcoal if you have the draft door wide open, but that will be accompanied by a lot of fire and a pegged dome temp gauge.
What was the last time you checked your dome temp gauge?
Btw, I go about 20 hrs on my butts too. Usually two or three of them, 7-8# each. They'll take at least 20 hrs at 225f.