Our smoke detectors are hardwired. One quit working and it caused all the detectors to go off as if the house was on fire? Why did it do that?

Thank you for reaching out. Hardwired interconnection uses the wiring in your home to interconnect AC-powered alarms together. The alarms communicate with each other via DC signal in the orange wire, which we call the interconnect wire. The same fuse or circuit breaker in the home powers all interconnected units.