Blockchain Hack, this one is impossible unless you have 51% of the entire mining machines which no one knows how many machines there are, or impossible for you to have it.
Government Ban, this is the most possible thing to happen to stop bitcoin, specially if they ban bitcoin mining.
Major exhange hack, already happened in thew past but did not have that much effect. Not all people store their bitcoins in one exchange or even not all people are trading their bitcoin.