Supply and demand, like a true open market should be.
Discussion
I don't think there would be much supply though. Bitcoin holders don't like risk. They would only be willing to make collateralized loans, which are low/no risk, and only people with collateral will get loans, which creates a lot of inequity. We need to be able to take some risk or humanity will stagnate.
Also what I described is a true and open market for interest rates.