DIAGNOSIS Smoke is backing up into the hopper. This is a vacuum/airflow issue. The combustion fan isn't creating enough negative pressure to pull smoke through the grill and out the stack, so it finds the path of least resistance back into the hopper.
FIX First, check the absolute basics: is your chimney cap open? It should be fully open, not closed down. If that's fine, the combustion fan might be failing or obstructed. You'll need to access it behind the control panel. Listen for a weak hum instead of a strong whirring sound on startup.
IF THAT DIDN'T WORK A massive grease fire in the main barrel can also cause this by creating more smoke than the fan can handle. If you see flames in the barrel, shut it down and deal with a serious cleaning.
HEADS UP This is a potential fire hazard. Never leave the grill unattended when this is happening. Pellet grasses can smolder in the hopper if smoke is getting back there.