Interesting. That was posted by me more than 3 years ago. I never did find the exact reason for the back draft but the next year Candy Sue was doing an overnight cook on the FEC100 they have at the pellet plant. Like mine the unit is inside with a stack venting outside. She returned in the morning to the same deal, OT kicked and pellets in hopper smoking and charred.
I did notice on my Louisiana Grill that they lift the pellets up hill from the hopper in back and then drop them down the chute to the firepot. I haven't seen the design of the new FEC but maybe they are doing that also.