Run Away Temp

Today, I wanted to smoke some salmon @ 150 degrees. I cleaned the firebox, then started the smoker specifying the 150 degree temp. I came back about a 1/2 hour later and things were chugging along at 145-150. After I put the salmon on the top rack, I left it alone for about 45 minutes. When I came back to check on things the temp was 452 degrees!
What might I have done wrong here?
Thank you for any suggestions.
Ken
What might I have done wrong here?

Thank you for any suggestions.
Ken