I don't see much of any "reason" behind what this invading army has been doing for the past 3.5 weeks, actually. Or if it's even right to call them an army.

Anyway I don't think the concern is that they would target or otherwise intentionally sabotage the reactors -- but that nonetheless they would allow something bad (potentially seriously bad) to happen by pure neglect. Kind of like the way they managed to blow up Reactor 4 in the first place back in 1986.