I think the carb ice theory was always going to be the most obvious cause. I have to say the pilots action of using carb heat on the downwind leg only seems quite bizarre. I have met several instructors through the years, and every single one has used similar methods. Carb heat on throttle reduction to 1700rpm when on the base leg, that carb heat only being stuffed back in on short finals, thereby giving full power if you need it.