I'm not sure, but I think the cracking came first.

I started because of a Bill Nye the Science Guy video we watched in my class in fifth grade. There was a short section on knuckle cracking, and what makes the noise, and I tried cracking my own knuckles. I haven't stopped since, and that was 8 years ago. Fairly recenty (last year or two) I began cracking my toes, though I don't crack then nearly as often as my knuckles. Occasionally I feel the need to crack my knees, but that's even less common.