Probably the usual, they were either compromised to serve up malware or their mail server was compromised to serve up phishing emails and the like and the block

Probably the usual, they were either compromised to serve up malware or their mail server was compromised to serve up phishing emails and the like and the blocklist hasn't been updated since when they fixed the issue. To the site owner, contacting Barracuda is sufficient to get you off the list if you're clean.