Definitely not randomly chosen--Apple would have chosen people it believes may have evidence that relates to the case. It's a legal request to preserve that data.

But it doesn't follow at all that Apple is threatening to sue them. A long time ago, in an unrelated case, I got a letter like this because I was in the room when a certain decision was made and happened to have some notes about that meeting. But there was no chance I would be sued. I wasn't the decider, and was basically a third-party involved.