The reasons vary, but the scene that plays out is almost always the same.

Bank customers get a letter in the mail saying their institution is closing all of their checking and savings accounts. Their debit and credit cards are shuttered, too. The explanation, if there is one, usually lacks any useful detail.

Or maybe the customers don’t see the letter, or never get one at all. Instead, they discover that their accounts no longer work while they’re at the grocery store, rental car counter or A.T.M. When they call their bank, frantic, representatives show concern at first. “Oh, no, so sorry,” they say. “We’ll do whatever we can to fix this.”

But then comes the telltale pause and shift in tone. “Per your account agreement, we can close your account for any reason at any time,” the script often goes.

These situations are what banks refer to as “exiting” or “de-risking.” This isn’t your standard boot for people who have bounced too many checks. Instead, a vast security apparatus has kicked into gear, starting with regulators in Washington and trickling down to bank security managers and branch staff eyeballing customers. The goal is to crack down on fraud, terrorism, money laundering, human trafficking and other crimes.