These are email addresses that are expired, or that never were 'real' recipients that Spamhaus acquires from ISP's. They re-purpose expired domains and rumor has it - also plant addresses on various websites around Etherspace. Since these are not 'real people' - the addresses should never end up on an opt-in list, so if you send an email campaign and it ends up in one of Spamhaus' inboxes - clearly your list development practices are not cool.
Some list vendors develop emails lists - albeit illegally - by scraping websites for email addresses. This is why you should never us these lists]. Spamhaus then adds the sending email servers to their blacklists. Overall it's a pretty good system but not flawless in our experience. For example, if you are capturing registration information from your website or from online events, an ill-willed deviant can enter a bogus / honeypot address into your list. Your well-intentioned campaign gets caught and viola -
you are on Spamahaus' list. Solution: Always email list use double opt-in processing (most email services providers like Pinpointe provide mechanisms to enforce double opt-in when using their forms to collect subscribers). UCEProtect. UCE Protect deserves mention because its one of the few major SPAM blacklists where you can blacklisted because of something someone else did.