If it happens again, it might be worth doing a lookup to see if your home IP address has appeared on Spamhaus' SBL at the time.
Spamhaus do real time blacklisting with IP addresses being added when multiple detection events occur (they don't just rely on a single detection) and then automatically removing the listing when no detection events occur after a given amount of time.
If you are ending up on their CSS you'll need to start doing some homework as to why.
You can get your public IP address by typing what is my ip into a Google search. Once done have a look here: