A few reasons may be you were not served properly notice of the lawsuit, maybe you moved. Why the case would have been dismissed had you shown up in the first place or maybe the debt was not your or you were unaware that their was legal action against your, or you already paid, Basic ally any number of reasons can be used.
There can be a number of reasons - it would be better if you provided us with some useful information about why you need to have one vacated rather than ask us to list all possible reasons. That being said reasons would include never being properly served or perhaps being in a hospital during the time in which the default was taken against you. That being said, once you have passed the initial 30 days it can be difficult to get one vacated and there are certain elements you have to establish in order to do so - for this reason and others you should speak with an attorney before proceeding on your own.