This particular scheme has been a ridiculous plague among my circle of friends on instagram recently. People create accounts mimicking an existing user, add an underscore at the end of the username, and then spam follow requests to all of their connections. Most people get a notification from someone they know, and they accept it without even thinking about. It is insanely effective.

Reporting the accounts for impersonation seem to do nothing, instagram's responses to the support requests even say they don't have enough people to look at all of them, and so they didn't.