There are two ways to get followers in Snapchat , using username or using Snapcode. Mostly people search username to to follow someone.
If someone adds you by Snapcode it is likely the case that they found your code somewhere on the Internet, screenshotted it and added you in the Snapchat app. May be some one from your friends or family member shared your Snapchat code on the internet.
So it is possible that social media can lead to you getting added by numerous people you don’t know if you don’t want this to happen please don’t share your Snapcode online.