It’s easy to get on del.icio.us popular and drive a bunch of traffic to your stuff: just set up various fake accounts and all bookmark the same thing at once.
If you don’t notice why it’s suspicious, look at the first ten+ people who have bookmarked it, they have just joined in the past 2 days and have all bookmarked similar things.
I’ve started filtering some del.icio.us spam from LiveMarks, but this kind of thing is hard to stop automatically.