I think that part of it was because people knew that nerds were kind of on the bottom socially, and currently being viewed as a victim is very trendy so people wanted to be part of a group that they viewed as victimized and ostracized. Another reason is because nerds are associated with intelligence and being eccentric, and people without a personality see it as a label to somehow acquire one for themselves. Similar to how it became trendy and fashionable to call yourself depressed or to have a plethora of mental illnesses. A bit of a dated example at this point, but a lot of people want to be looked at as being bRoKeN like Kurt Cobain but without actually being truly fucked up like they imagine the "tortured artist" to be.