We don’t know. There are various ways in which this could have happened: rogue human actors at the controls; injection of a false launch instruction into the chain of command; or compromising hardware/software with built-in vulnerabilities from the outside.
We’ll likely never know, as the details relate to highly classified systems and processes.
If the naval collisions and Mil airplane failures are any indicators, the source is probably control system compromise and intrusion.
Whatever the answer, it doesn’t indicate an encouraging picture of the state of our national defense.