Probably wrong, but if you have a VERY tight house then maybe you are generating a slight vacuum inside your house from pulling all that air out. Then, at a certain point, something gives, like your doggie door swings open or something, letting in air all of a sudden to relieve the neg. pressure and the fire whooshes up toward the flue. This is probably lame brained, maybe it is something to do with inadequate gas mixing with air prior to combustion. There must be a spot where the air is supposed to enter the burner, along with the gas. To test my notion, leave a window cracked open to see if it stops it, also, maybe it won't do it with the glass doors closed.