There are many reasons why a market goes down, but the main reason is because it has gone up. The market, so far as I know, has always ratcheted up and fallen down. I don't expect that to change in my lifetime, nor do I expect a change in the long term rising of the market due to inflation in a land of fiat currency.
I voted "other" because I think it will be because of a combination of the choices as well as: 1. The economy is not really doing that well, it just looks that way because it is being compared to recent dismal performance. 2. When QE2 ends or is discontinued, the results will be a downward force on the market. Subsequent QEs will only serve to further devalue the dollar and eventually bring about an even weaker economy. 3. The investing public is snake bit due to the 2008 crash and will get skittish quickly when this market starts to turn around. 4. There is a good chance that our fiat currency is on it's last legs. This does not bode well for the average person using dollars to pay for basic commodities. 5. Our government has a lot of fiscal problems to correct before a strong market can be sustained long term. 6. The government has propped up a lot of weak players in the economic landscape over the past few years and they have to be washed out and replaced before the economy can truly correct itself.
In the last 18 months we could well have had a 30% bear market. Afterwards we could have attributed perfectly logical explanations and rationalizations as to why the market had to go down. But it didn't. Reasons will always be made up after the top/bottom.