the only time my bud does this is due to near microscopic little seeds. i get it a lot on longer flowering strains, even if i never actually see any nutsacks. little tiny seeds that could probably be mistaken for bugs but when the herb is broken up and i look real close i can identify them as teeny little seeds in what look to be newer (smaller) calyxes than the rest of the bud. i don't buy the whole chem-crackle deal. i never flush and i never have this problem unless i have little (almost invisible) seeds. i've not flushed with foxfarm, jack's or dyna-gro, and never had the harsh crackly smoke without seeds.
weed that was quick dried or poorly dried/cured/cared for after harvest can crackle too, but that seems just due to there being more plant matter and less even moisture content, and it's not quite the same as the tiny seed crackle.