[piaste]

From what I saw, "alt-right" had existed for a whole in blogs and webzines, but it really became popular during the Republican primaries.

The Trump and (early on) Carson supporters needed a term to distinguish themselves from the established Republican base, consisting largely of neocons and evangelicals, with their rather different ideological concerns. "Alt-right" offered the vaguely young/edgy and sans-culottesque character they needed.