Ive seen it happen also but never the whole muffler. Mabye the muffler is too small for the engine?What kind of muffler do you have installed? How new is it?
An older/rusted out spark-arrest muffler may display a slight glow after a long run of the lawn... Although I've never personally encountered this -- just an assumption. I've seen sparks shoot out of them before and even seen elements on the inside of one (after dark) glow a reddish color. Never an entire muffler though.