What causes non-existence?
In ancient times, if you wanted to completely destroy somebody, you'd wipe their name out of history. For example, there's that one guy in the Bible who, when the Ark of God began to fall from the wagon, he stretched out his hand to catch it, and God smite him...what was his name? That's right, we don't know, for even his name was to be forgotten...as if he never existed. This was also the case for many among enemies. Sometimes we'd only know them for their titles, and sometimes they aren't even mentioned at all so as to wipe them out of remembrance completely.
Although, if you're asking what causes non-existence, as in as if you really never existed, then my answer would be "abstinence", for if you were never created, then you never existed.