I have this happen to me pretty often when my flue is cold and I start a fire and take it up to temp really fast (like stone cold to 500 in 5 minutes fast). The crackling sound is a thin layer of creosote expanding at a different rate than the flue pipe and fracturing. Sounds like candy wrappers. See a few black flakes come out of the cap...you can really tell if there is snow on the roof/ground. This isn't necessarily a bad thing - IF - that's whats going. Could have been a small chimney fire.
This is why I clean my wood stove flue twice a year. Family and home are far too important. Just a few hours, gives great peace of mind.
I do not think it necessarily was a quick flash fire that caused what you heard and saw. It can happen when the flue warms really quick; remember that metal, like water, changes a bit when warmed or cooled. Starting a quick fire causes the metal to expand slightly which can flake off a thin amount of that black stuff and send it on up the chimney. This is what happens when you find that black crap on the lawn later. Again, not necessarily a fire but a quick warm up of the flue. I would not worry much about it but still would check the chimney.
I use Rutland creosote remover about once a week and those black flakes all end up in the bottom of my clean out.
Its purely a differential expansion as has been noted above. the flue is expanding and the brittle creosote cracks combined with a strong draft the flakes fly out instead of dropping down.
