Throughout our life cycle, both men and women experience specific developmental challenges relative to their age. These challenges start as early as the first few months of life(mastering motor skills) to the end of life(acceptance of diminishing capabilities). Depending on your religion or theoretical orientation, these stages could be broken into anywhere from 4 to 7 stages.

The reason this developmental stage is given more attention is the level of impact this crisis can have on others. Symptoms of struggles at this age have more impact on family and careers than other stages. There may be more historical references about men having these type of crises but as woman have taken on roles that were once reserved for men, we are seeing similar impact on families these struggles have taken. More attention may be given to a father who makes drastic changes in his life than an adolescent who is rebelling(a common response to a teenagers developmental challenge at that period). Woman may go through their own midlife challenges and present the conflict differently or the impact may be seen differently. It does not mean they are sailing through this period of life carefree.
The causes could be from external events(getting fired from job) to internal reaction to a situation(having a good friend have a heart attack). It is important to understand that some struggles at different stages in life are normal. What we could define it as "crisis" is relative to the situation and how this conflict plays out. Having a support group to use as a reference point or sounding board are often helpful. Talking concerns out with a trained therapist could also help, especially if decisions are being made that have significant implications.