I was doing a cook today...I was at 280* on the PB set temp and reset it to 400*. As I sat there, the actual temp just kept dropping and dropping. Finally, I shut it down, pulled everything out and checked the firebox...it was full of pellets! What happened?
How many cooks did you do before this one without cleaning? This happened to me after a few over nighters when I fired up the grill for some burgers. I cleaned out the firebox and the entire lower cavity with a shop vac. All worked well after that.
I've had the same thing man. I hadn't cleaned my grill for who knows how many cooks and like TomC said I cleaned her up and she fired right back up with no issues.
With the longer cooks you get ash build-up. The ash takes up space in the firepot and reduces the air flow causing a low or no flame condition. So in your case when you bumped up the temp and the MAK started dumping pellets in it is the pellets that actually put out the fire. After long overnight cooks you will want to vacuum out your firepot of excessive ash. I vacuum mine out after every cook so I have no issues. It just takes a quick second. I keep a small shop vac handy for just that purpose.
Depending on what kind of pellets you are burning you could have had a lot of ash in the firepot after the all nighter. You didn't state how long you had been cooking at 280 when you pushed the temp to 400 but you added additional ash while at 280 and when you pushed it to 400 the MAK most likely continuously dumped pellets into the firepot to attempt to get to and maintain 400. At higher temps the MAK auger pretty much runs continuously.
The mechanism that causes this a firepot with a bunch of ash in it and your finger setting the temp to 400 without cleaning out the firepot.