There are a few possibilities. One is that someone has a username similar to yours and keeps trying to use your username with the wrong password. Another is that your email was included in one of the recent hacks (of Adobe, MacRumors, etc.), and enough people are attempting to use whatever password you had on those accounts (with minor variations) to access your Apple accounts.
Either way, there's not a lot you can do. Apple can see who tried to access your account and triggered these requests, but I don't know if they will help you do anything about it. Two-factor ID is one way to make access harder, but I don't know if it will cut down on these emails.