It wasn't a real on-budget surplus. It was only a surplus if you took the revenue from Social Security tax and ignored the obligation to later pay Social Security with it.
I think that this gets into what the deal might have been with respect to whether you count the social security tax or not. I think that they conclude there is a "surplus" either way, although it is of course smaller. However, you might be referring to something else.
I assume you are referring to 2000. I am very much a newbie in this stuff, but think that the "story" from the OMB outlays/receipts data (which is what this is based on) is only part of the story. For example, I read somewhere that some were arguing that this "surplus" in that period was not a good thing in some respects. There's a lot to learn.