It's because it's burning very very low quality coal, by the looks of things mostly coal dust. This would mean the fireman has to shovel almost continuously. The small size of the coal means it burns very quickly, and is so light that the exhaust (when working even remotely hard) sucks the still burning dust through the boiler and out the stack before it can even burn completely. Since it burns so quickly and is sucked out the stack even quicker it doesn't provide as much heat as a regular lump of coal, which means the fireman has to add even more of the bad coal. This can snowball if the engine is worked hard for long periods on nothing but dust. For reference the ideal size of a lump of coal for firing a locomotive is about the size of a fist. The coal that's being burned by that engine is probably similar in size to sawdust. With the coal being so poor the ash has probably solidified into clinker, either in the ashpan or worse, in the firebox. Clinker (being a metallic substance formed by a chemical reaction) is too heavy, large, and solid to be lifted up and sucked through the tubes to exit the stack.
-Ben