I bought a house on a mortgage. If its price changes later, how will it affect the ease of repaying the loan?
If a mortaged house decreases its market price after the mortage was made, the bank is in risk in case of default - it might be unable to recover the amount it loaned. In practice this doesn't change the amount you owe to the bank or the monthly payment -  you owe the same regardless of the value of the house after making the mortage. 

I believe the author meant that the people the banks were lending to would only be able to afford to pay the mortages if the economy kept on booming, and they would remain employed.