There are really two meanings for the term "crow hopping." A fielder is said to be "crow hopping" when she picks up the ball and takes that little hop while preparing to throw. That's a good thing—it helps the fielder throw farther by using their hip and chest muscles in addition to their arm muscles.
The second meaning of crow hopping involves a similar, but more subtle move by a pitcher making a pitch. "Crow hopping" by a pitcher making a pitch is illegal.
There is a general requirement that a pitcher not leap (both feet off the ground) while engaged in the pitching motion. A pitcher is required to keep her pivot foot (the foot she pushes off with) on the ground, dragging it forward as she strides toward homeplate. When both feet leave the ground it is officially called "leaping," although many coaches mistake this for "crow hopping." Leaping is not permitted under ASA rules.
When a pitcher actually strides forward (usually leaping in the process), replants her pivot foot and pushes off with it as she releases the ball, this is an illegal "crow hop." The reason this is prohibited is it effectively gives the pitcher an opportunity to pitch from a much closer range than she would otherwise.
In practice both leaping and crow hopping are difficult to enforce. Umpires generally issue a warning and leave it at that.