MACON, Ga. — H. Averett Walker used hot money to turn Security Bank from a sleepy Southern lender into a regional powerhouse. Darrell D. Pittard used hot money to jump-start his brand-new MagnetBank, allowing it to lend hundreds of millions of dollars even though it did not have a single drive-up window or even a customer with a checking account. It is a formula being replicated at banks across the United States. Rather than simply wooing local customers, they have turned to out-of-state brokers who deliver billions of dollars in bulk deposits, widely known as “hot money,” from investors nationwide....