Your IP may have been added for a number of reasons.
We use multiple data sources checking millions of IP addresses every day to keep our clients websites secure from spam and hacking. Our system considers a number of different variables including historic data before it considers your IP as blacklisted.
Spamming, Abusive Behavor, Attacks on Websites, Is Bogon, Cloud Provider, Is A Proxy, Is a Relay, Tor, VPN (Often, VPN IP addresses will be black listed), Anonymous, Classed as a Threat, Blacklisted on other blacklists, etc.
This list of reasons is not exclusive and many other factors are considered when adding an IP address to the blacklist.