If the price is under $1, the backer can purchase the coins with reserves and pocket the difference. The price will only depart from the peg if selling forces the market price down and the backer doesn't have enough reserves to purchase tokens sold beneath the peg. This scenario is similar to a bank run and would require high selling volume to set if off.
The lending rates are determined by a separate mechanism and reflect the probability of a future departure from the peg.