The problem is that falling house prices make it harder for those average people on average incomes to get mortgages.
Let's say that someone wants to buy a house for �100,000. The mortgage company need to know that, if the buyer defaults on the payments, the repossession value of the house will match the debt. If house prices are rising, the mortgage company will know (or at least guess) that the house might be worth �120,000 in a few years time. So they'll consider offering close to a 100% mortgage (perhaps 95%) because they know that their money is safe.
However, if prices are falling, the house might only be worth �80,000 in a few years time. Lenders won't loan more than the value of the property upon which the loan is secured, so they might only offer a mortgage of around 75%. That leaves the buyer needing to save a �25,000 deposit before he can get a mortgage.
Chris