Why will your XR100 blow fire when you let off the throttle and the bike idles down?

The carburetor is allowing unburnt gas into the exhaust manifold. The presence of high heat and cylinder combustion ignites the fuel inside the exhaust and is typically seen coming out the pipe. The less restrictive/no baffling an exhaust pipe is the more pronounce the flame and popping. All engines do this in some degree, but excessive flame points to an overly rich fuel mixture.