Why is the IP address blacklisted?

In simple words about suspicious activity, "blacklists" and possible reasons for getting your IP address into them.

Blacklists are databases of suspicious IP addresses. These can be addresses from which found suspicious activity (like spam, DDoS attacks, and other illegal activities). Often the IP addresses of bona fide users fall into blacklists. For example, several recipients of your company's mailing list complain about spam, or the mailing does not comply with the rules of postal services - this may be a reason for placing the address on the blacklist. In addition, these databases often include addresses for testing.

Different blacklists use different algorithms for detecting "suspicious" addresses. Therefore, the IP address may be in one database and not in another.