Well, I guess I'm one of those former long hair relaxed heads.
Well, I can only speak for myself, but my reasons are pretty basic.
1. Health
2. I want to experience my natural hair (been relaxed since a child)
3. I like my natural hair (but definitely miss sleek styles)
But overall, I think many relaxed heads are trying the natural route, because society is changing along with the hair industry. Many realize that they can achieve the same sleek styles without using relaxers and tons of grease for a press. And thanks to wigs and extensions, many are able to transition without worrying about styling our hair, so it makes it easier.
Also, many people and job markets are alot more accepatable of natural styles. It's not as taboo as it used to be, to wear twists, dreads, braids, big hair, etc.
I think many naturals decide to go back to relaxing, because they truly understand the concept "It's just hair". They know that it will grow back and they can decide to do whatever they want at any time. And of course because of manageability. I'm sure the extra care and time that's needed to work with natural hair may start to wear on a person and then they decide to relax. It just depends what's best for you at the time.