What could cause power to go down just by moving?

I have just seen an Invisible Inc. mission fail because POWER was lost and I can't explain why.
An agent was in a tough spot between a drone and an armoured guard. Having already rewound we knew the guard would see the agent on their next turn. We worked out that there was enough AP to move out of sight of the drone, shoot the guard, hack the drone and then get past it. There was only just enough power to do this though, and after moving into a cover position where the guard could be seen and shot, power had dropped from just enough, to not quite enough to hack the last firewall on the drone.
I can't think of any mechanism which could have caused the power to be lost like this, so is there something I'm missing?
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