Sorry, I should have been more clear. I mean that, from my knowledge, UCSD has chosen their first round of applicants they want to admit to the program. But, if any of those take another offer, then it'll trickle down to the wait list. I don't know necessarily whether UCSD tells people that they are on the waitlist or not though. It's also worth keeping in mind that the selection committee for incoming grad students isn't made up of the entire composition faculty -- a few take turns each year. So, maybe you've had prior interaction with one of the professors -- they may or may not be on the committee to view your application that year.
Hope that clears it up a bit.