There could be many reasons that user shows interest to unsubscribe email alerts from your website. Users who shown interest and confirmed their subscribtion using double-opt-in system may also unsubscribe from your service if you don't care about users' interst. Assume that you have serveral services from your company and user has shown interst to receive emails for a particular topic. When you send emails of inintentional topics to user, he / she does't care about your email campaign. Insted the recipient frowns up on receiving your email and tend to click on unsubscribe links. 

Here are two top most reasons because of which users unsubscribe from your campaigns:

1) You’re emailing them too much

2) You’re not emailing them enough

3) Your emails aren’t relevant

4) Your emails aren’t mobile-optimized

5) Your emails are too long

6) Your content is repetitive or boring

7) The same information is available online