Hacking into military drones, as in ‘The Oracle Machine‘ graphic novel, might seem implausible to some, but there is historic precedent of such incidents.
In 2011, the famous U.S.A. Creech Airforce base suffered a security breach, when a keystroke logger virus infected multiple machines. (Read about it in Wired, or The Guardian)
It’s thought removable hard-drives ‘breached the gap’ between classified and public networks. So maybe a drone pilot was loading up some music downloads to fly with?