It's kind of a spin off of the phrase "granola" referring to what used to be "hippie" ideals- breastfeeding, using all natural home products, wearing your babies in a sling, etc. MDC has 2 different major goals- "natural family living" (no chemicals, reusable everything instead of disposables, overal environmental awareness) and "attachment parenting"- breastfeeding, babywearing, attending to babies at night, gentle discipline/no spanking, etc. Both are described as "crunchy".
Somehow "granola" morphed into "crunchy granola" and then just plain old "crunchy" (with variations such as chewy or crispy or soggy for people who are only partially living the "crunchy" ideals.)