Bubbles.
They tends to explode.
I remember this same conversation about a year and a half ago when Bitcoin spiked from forty cents to parity with a dollar. There was no reason in the world for it to be valued so high and that it was a bubble and would collapse.
It did drop 40% after ($1.10 to $0.60 ish) but I'm still waiting to buy back the bitcoins I had sold at $0.50.
What could happen now?
If bitcoins are truly determined to be a form of "prepaid access" which can be spent internationally and is sold to those in the U.S., then FincCEN might be of the opinion that no matter the amount, the seller needs to collect identity. If they were to state it clearly like that it would definitely set the mood.
There could be some widely used software that sends out an automated update and the update happened to have a wallet stealer buried deep inside, thanks to an automated build and release system that a hacker was able to slip in a few lines of code without anyone noticing. That would not be a good thing for bitcoin.
Here's another scenario. The block reward drop could come, and the price stays at this $10-ish level. People that bought assuming it would go up from here, start selling, and it is back to single digits, which gets exacerbated by the momentum traders. Especially once a method to short bitcoins on margin returns.
Perhaps much of the demand is coming from ponzi victims running out and buying bitcoins and handing them over. When the ponzi ends, not only will that demand for bitcoins drop off, but the ponzi-master spends them suddenly to lock in the value at a higher price.
Lots of things to legitimately be worried about.
That's why buying bitcoins to hold cannot be considered anything but speculation. But for a $100 investment, the downside is $100, the upside potentially could be a lot more than $100.