The book, Caliban's War, doesn't shed more light (heh) on the issue.
The mirrors do, as you assume, orbit Ganymede. At least: Almost every time they're brought up in the book, they're called "orbital mirrors".
The great orbital mirrors had always been his allies, shining down on his fields like a hundred pale suns.
Caliban's War, page 21
The following passage might imply that the mirrors start falling when the power is cut (how long that takes is, like in the show, not stated):
Power to the grow lights had been cut, and the mirrors … He couldn't think about the mirrors.
Caliban's War, page 25
If there's an actual correlation between the power failure and the mirrors falling, this might mean that the mirrors normally experience orbital decay, and that constant adjustments to their course are needed to keep the orbits. When the power disappears, Ganymede's communication with the mirror array is lost, and the mirrors' orbit starts decaying until they fall down.
In order for this to happen, though, Ganymede would need to have an atmosphere. Orbital decay like this would really only occur if there was atmospheric drag. However:
The atmosphere was so thin it could pass as an industrial vacuum.
Caliban's War, page 9
I could only find one other person raising this point online. The authors and showrunners do not seem to have addressed it, and I couldn't find any articles talking about it either (although there are many articles about the show's adherence to real-life physics and the few holes that do exist, I couldn't find any about this).
All in all, this seems to be a plot hole which hasn't been caught by a lot of people.
user20310 said the following in a comment:
My assumption was always that the explosions pushed some into higher orbits and some down out of orbit
This might also be a possibility. Prax Meng often talks about that "the cascade" will kill Ganymede. He means that the moon's artificial ecosystem will collapse, piece by piece, because of the events at the beginning of the story. However, small events cascading into larger ones is a theme throughout the book (and season; episode 10 of season 2 is even called "Cascade"). It's possible the authors intended the destruction of the mirrors in orbit to trigger a Kessler cascade. However, with the low atmospheric drag, this wouldn't suddenly cause the mirrors to lose orbit entirely, like you say in your question.