Most of DeFi lending systems are based on the Money-Market concepts. The interest is variable, and the loan term is variable too. This means the borrowers can choose when they pay back their loans, and lenders can choose when they ask back their funds from the joint lending pool. However, the lenders can receive their funds only, if there are enough free funds in the pool. In case of adverse market situations it's quite possible that there are no free funds in the pool, therefore the lenders cannot withdraw, and hence the bank run on the Money Market.