Smoking - Why does some crack catch fire?

[jdrug]

Though most decent rock just vaporizes into the screen, occasionally swim finds some rock tends to catch fire and create a flame at the end of the pipe (assuming a glass tube and screen). What causes some to catch fire? Is it a cut of some kind? Something in the preparation process (too much baking soda, etc)? Possibly just wet?

...or is this just another case of "it could be anything"?