There's a psychologist doing work on this right now. I'll try to dig up the links.
My suspicion is that the brony phenomena is linked to being in the right place at the right time in the consciousness of a set of young adult males. I think there's a lot to be afraid of as a 20ish male, consciously or unconsciously. It begins with 9/11, which I think is a trauma that has yet to be properly explored in the consciousness of the nation's youth. Add on the crippling economy that most of us have to face and the traditional expectations of society of males of this age, combined with total uncertainty about the future, I think MLP takes advantage of this subconscious state. It defies maleness, adulthood, and complicated realities at the exact moment that a subset of the population wrestles with those issues. I don't know if it's a healthy coping mechanism or not, but most bronies I know are successful, well rounded people, which suggests to me that it's at least not unhealthy. Some fans take it too far, sure, but there's nothing in existence that hasn't been taken too far.