Why does some marijuana pop crackle and emit sparks when smoked?

It does this because this weed your getting is picked too early and it still has seeds and when you burn these seed's they often pop, blow up or emit sparks.

First off if the marijuana has seeds in it it is because it was pollinated by a male plant and not because it was picked early. Second it sparkles and crackles and pops because it was not flushed properly before being harvested so there is fertilizer still in the bud.