Problem with YS640

Not real sure where to post this, so here goes.
The last time I used my 640 I used it with the Grill Grates(?) for doing a Steak. Set the temperature at 450, let things heat up well and proceeded to cook up an excellent steak. Shut off the grill, went inside and had dinner. Then I went out to unplug the grill only to find that I now had a fire going in the Pellet Hopper. Not a good thing. Worst part is that this has now happened 2 of the last 3 times I have used the unit, Both times I was using the Grill Grates and had the temperature set at 450.
Unit is 4 years old and has turned out lots of great meals, just seems that either something is wrong with the unit or suddenly I'm doing something wrong.
Any ideas?
Joe
The last time I used my 640 I used it with the Grill Grates(?) for doing a Steak. Set the temperature at 450, let things heat up well and proceeded to cook up an excellent steak. Shut off the grill, went inside and had dinner. Then I went out to unplug the grill only to find that I now had a fire going in the Pellet Hopper. Not a good thing. Worst part is that this has now happened 2 of the last 3 times I have used the unit, Both times I was using the Grill Grates and had the temperature set at 450.
Unit is 4 years old and has turned out lots of great meals, just seems that either something is wrong with the unit or suddenly I'm doing something wrong.
Any ideas?
Joe