There are many examples of words which have changed from one usage to the opposite usage over time. Usually these start out as a literal usage, and develop the opposite usage as a result of being applied to an individual or group that gave the characteristic "a bad name".
For example, a "pedagogue" was once simply a teacher (greek: paidos "child" + agogos "to lead") and took on the meaning of "strict disciplinarian" as a result of certain practices once rather common in schools.
Expanding on the answer given by Fr0zenFyr, when the word was first coined, it was probably applied to those who stood out in a way that was admired; then, at some point, either those qualities became undesirable, or some notable person who was thought of as standing out from the crowd did something unpopular, or it became undesirable to stand out at all, or people starting paying more notice to those who stood out negatively rather than those who stood out positively, or... (the possibilities are endless, but it almost always end up to be some sort of social phenomenon).
If you feel like researching this more and really get to the bottom of it, your best bet would be to do some cross-referencing between literature and history, and see if you can root out any correlation between the change in meaning, and the prominent people and events at that point in time.