That or lost connection/battery.

The only wireless controller I ever owned would continue any repeating input active when connection dropped, if batteries or wifi died; they could just be cruising at max speed off in whatever direction they were facing, until the motor batteries run dry.

You'd like to think they'd carry backup interface, or even just use a wired controller, but this whole op sounds pretty McGuyver top to bottom.

Pretty unlikely. It's easy to dunk on them for the controller, but they apparently carried backups, and it's nowhere near the most concerning thing about their operations. It's much more likely that their extremely brittle carbon-fiber hull fatigued (again), their homegrown acoustic fatigue detection system didn't detect it (or it did and they ignored it), and the ship was crushed in a fraction of a second.