I'll pose a scenario. Keep in mind this is speculative; it may never happen, that's not to say it can't, and it's not to say there aren't other routes for which this can happen.

As many of you know, supply and demand are proportionate concepts. One creates the other. A void in one creates a void in another.

If you've studied history, you know that the Wall Street crash that caused the Great Depression was caused by investor speculation that a mass stock drop would occur; they inadvertently caused it.

Bitcoin could go the same route, although by a different impetus.

As we all know, the difficulty is skyrocketing to the point of mining being absolutely inconceivable except by those with IMMENSE power.
There COULD (not saying will) be a point where the difficulty increase becomes so much in a single period, that it causes a significant amount of people to say "fuck this, I'm out". Those people may sell off their BTC, which if enough people decide to do so, will create a size-dependent drop in the value of BTC at that moment. At that point, the sell point is mainly at the mercy of the domino effect - if that drop puts enough of a fear of God into people that the value will free-fall, they'll do the same, and create a situation that if not curtailed by holders that recognize it, is much like a steamroller going down a steep hill with a faulty Jake brake.

It's unlikely and almost inconceivable to get to $5, though, because the main benefit of having a large, distributed network is that it takes a sizable amount of people to sell out to drop the value. This is inertia of a stock. You can better believe for the number of people panicking and selling off there will be an equal or greater number of people holding on to it, and that's why I can only see it dropping 50%, maybe 75% at max, in true market free-fall conditions.