No, I don't believe it is just the 9XXX Deere's that are burning up, there are red and yellow combines catching on fire too. I don't think motor location means a dam thing either.
If you look at how much more crop material and trash the class 6,and 7 combines are taking thru themselves compared to the old class 3 or 4 machines, it is no wonder they catch a fire once in a while. Although, I had a field fire with the 6620 series Deere when a feederhouse bearing went out droping hot pieces onto the dry cornstalks. Used to have a lot of fires on the old MF combines. I just think fires are a fact of life, especially when such huge machines are threshing up such large swaths in the crops.
An effective optional fire extinguisher system should be offered to cure this, as I think it would be considerably cheaper that than to design and maybe impossible to design a dust-proof combine.