One way that can bring down bitcoin just like any market is by shorting that market too much or a typical pump and dump. Investors would lose interest, market would plummet, bubble will pop.
Well I think its not truly possible now, yes one can push bitcoin down for sometime by introducing new projects and currencies to digital world. But they could only give them a tough competition but would not be able to replace bitcoin. It is a promising currency and it is to be accepted that up and down is the part of every currency