This issue recently started, and I'm not sur why. Originally I thought it was an issue with the lump charcoal I was using (used it for the first time ever), but I used briquettes last night and the same thing happened. The flame is reaching the top of the charcoal, which luckily wasn't very high since it was only an hour and a half cook, but it's happened on longer cooks where the hopper was more filled.
The hopper is sealed, even with aluminum foil at the top lid, and the door is completely closed for the ash bin. I only had one wood chunk in it, and have never used a split in my previous cooks.