Just some plausible speculation:
1. DCM system is a retrofit, not part of the original design. Thus, possible that one/some function does not have redundancy.
2. The most commercially-viable DCM design uses highpower laser pulses to "confuse" an incoming missile. Plausible that electrical system feeds the DCM (as opposed to DCM having it's own generator), and one device acts as the gate for the airplane-generated electricity to feed the DCM.
3. It is likely that a highpower laserpulse system would be in an inactive state on takeoff and landing; both as a safety measure so close to habitation, and to protect the DCM since it is undercarraige mounted. Possibly, putting it into inactive/landing mode triggered the failure.
I am the first to admit that is speculation at worst, extrapolation at best. But I do occasionally have flashes on insight; "accidental access to the Akashic Record" as the Buddhists would say.![]()