When you put a bunch of small splits over a hot coal bed, it tends to out-gas all at once which often leads to a runaway situation where closing the air down no longer controls the sudden heat generation. Heat is being generated faster than the stove can dissipate it. EPA stoves are all designed so that air is never completely shut down. It doesn't even have to be small splits. Just avoid putting too much wood on a hot bed of coals. There are some threads on here about over-fire situations and how to load to avoid them.