When the National Highway Traffic Safety Administration determines that a vehicle or component contains a safety defect or fails to meet a federal motor vehicle safety standard, the manufacturer is legally required to notify owners and fix the problem at no charge.

Source: https://dmvct.org/recalls-safety/how-nhtsa-safety-recalls-work/

This site states they can also be issued if the vehicle doesn’t meet a certain federal safety standard. There have also been recalls for vehicles that have been fixed OTA with a software update so a physical defect is not a requirement for a recall.