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One of the well known problem areas with the 700/900 electric seats is the drive cable shafts between the motor and the track gear boxes being a bit short for the length of their cable sleeves. Basically the squared ends of the cable shaft don't always stick out far enough to stay engaged. Even though the cable shaft ends are flanged they can still shift back and forth a bit in their sleeves. When the cable shifts far enough, one end can disengage from it's drive cog. The motor trying to move the seat back and forth will then only move one track, obviously putting the other track out of synch. I highly suspect that's what's behind your problem.
The cables can shift during use plus the squared ends can start to round a little bit from wear. In your case, working on the seats recently likely caused a cable to move and allow an end to disengage. Operating the motor during testing would then put the rails out of synch. With the seat now back in position, if both sides now happen to move with the motor then the cable has seemingly re-engaged.
First off, you need to get the seat rails back into synch for equal travel. The rails should then properly align to their mounts. The simplest way to do that is disengage a cable from the motor and manually turn the shaft to move the track so the two sides are equal. Access is a bit of a pain. As I recall, I have the rear of the seat raised on a wood block and then tipped back tied to a rear headrest.
You then need to prevent this from happening again. In the 700/900 FAQ here, two methods are described. One is to carefully shorten the cable sheath so the ends stick out further. The other, and far simpler method, is to put a tiny 1/4" spacer into the cogs ahead of the squared tip of the cable at each end so the cable can't shift in as far and pull out at the other end. A piece of finishing nail works fine, filing the ends to length. As I recall, only the motor end really needs the spacer, but I think I normally do both ends so the cable is always centred. Do both sides of the seat forward/back motor to completely fix the problem. I inserted a feeler rod at both ends and as well carefully measured the cable to accurately determine the needed size of the spacers to centre the cable shaft -like others, I found 1/4" was just about right for the job. It can be a bit tedious getting the spacer inserted properly. I recall that tweezers helped. I put a dab of grease on the spacer to prevent rusting. A magnet or magnetic tipped screwdriver is handy in case you need to extract it.
I don't recall ever hearing that the other motor cables have this problem, just the track motor cables, and mostly the right side cable.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now
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