Trail Plug firing before leading at high load.
 

Can someone explain (in detail) why is it so detrimental to a rotary engine for the trail plug to fire before the leading while under boost. This could be unintentionally caused by a number of reasons like failing lead ignition or fouling/failing lead plugs.
Is not the trail ignition usually already firing usually in a retarded (safe) position relative to TDC? Is it because of where the flame front will be initiated relative to the rotor surface? I read something about the squish area and knock.

Please explain.