The explanation I've seen is that certain programs (mail, chat, forums, etc) automatically recognise urls that are not hyperlinked and convert them into a hyperlink.

E.g. "example.com" becomes "<a href="http://example.com/">example.com</a>".

Such features would now (potentially) also work on mentions of zip files, i.e. on messages that state "download install.zip from github".

Edit: ghi, HN actually does this. Not yet with zip TLDs though.