If you're getting a lot of new followers on some of your boards, and those followers look like they are new accounts, it’s likely that one or two of your boards have made it into our sign up process. These followers are legitimate (brand new!) accounts, and your account is secure.
When Pinners first join Pinterest, we suggest some boards for them to follow. The boards we show come from the Pinterest community, and the list is always changing. We think this is a great way for new Pinners to find great boards and Pins, and for everyone in our community to have the opportunity to be discovered and followed!