This is generally a sign that whatever you signed up with has been hacked, especially if the emails are shady (weird offers, spam, phishing, etc)
For instance, say you signed up on forumZZZ, and you gave out email of
ZZZ.word@spamgourmet.comNow you are getting random weird emails from someone you never heard of sending to
ZZZ.word@spamgourmet.comUsually means forumZZZ got hacked and they stole all the email addresses. This is also an indication you should change your password on that site, and any other sites where you have the same email/password (which is another reason why never to have the same on multiple sites).