Yes, if there was a sudden collapse in the price of STEEM, and no recovery in price, the peg may break. Quote from the white paper:

For every SMD Steem creates, $19.00 of STEEM is also created and converted to SP. This means that the highest possible debt-to-ownership in a stable market is 1:19 or about 5%. If Steem falls in value by 50% then the ratio could increase to 10%. An 88% fall in value of STEEM could cause the debt-to-ownership ratio to reach 40%. Assuming the value of STEEM eventually stabilizes, the debt-to-ownership ratio will naturally move back toward 5%.

Leon Fu