I originally began prepping due to peak oil. Our energy future is dim. Just to put some data to it, examine this table produced by the Energy Information Agency. Look at domestic production since 1970, Alaskan production since 1988, and total production all during that time.
http://www.eia.gov/emeu/aer/pdf/pages/sec5_5.pdf
That's one reason why people think the economy will crash--an energy shortage which will create chaos. Oil is in *everything* we do.
A second reason is the decline in the US dollar. The Federal Reserve is creating money out of thin air (fiat currency, i.e., backed by nothing), and the more dollars created, the greater the inflation.
Coupled with the gargantuan, huge, immense, titanic, pick-your-adjective size of the US deficit and total debt, there's no way to get out of this without trying to inflate the debt away. Inflation makes the dollar worth less, which means people want more dollars for what they sell you. Prices rise.
There are a ton of reasons why people think we're likely to have our economy tank. Debt, inflation, oil, an incomplete end to the housing crisis--pick your poison.
Here's a terrific piece that I think captures a lot of the concern, as well as the strategic response of individuals to what's coming:
How to Bet Like John Paulson
BTW, I teach in the renewable energy field, I had my students read the Paulson article and they were asked, on their most recent exam, this question: "According to Paulson, what is the one plausible route out of our appalling situation and why?"
You might wonder why in a renewable energy class we'd be reading this; the answer is that the value of the dollar greatly influences the price of oil, which relates to the economics of renewables.