Checking my saddle, the tension screw seems to have bottomed out to no tension applied. I added a small amount of tension to the screw, and the screw seems to have "taken." I've not tested if it will shake itself out, or unscrew by some other process.
1) Why would this happen? A failure in periodic maintenance on my part to check that it was staying in alignment?
2) What negative effects would this have had on my saddle?
Checking the tension on a Brooks saddle is part of the regular maintainance. Any bolt that doesn't have sufficient tension on it will move due to random road vibration.
From
http://www.brooksengland.com/getting-in-touch/faqs/saddle_maintenance/
Q: Why have the Rails / Tension Pin / Backplate on my Brooks saddle broken? By far the most common cause of breakages of the Rail, Backplate and Tension Pin is insufficient leather tension. Brooks recommend a routine six-monthly check of the tension, and adjustment where required.
A bolt stays in place due to the friction applied to the threads by the tension in the bolt. In effect you are "stretching" the bolt so it constantly pulls on the threads to create the friction to hold the bolt in place. In the Brooks saddle case the force is actually a compression, but the principal is the same.
As long as you put the tension back and check it on a regular basis, I don't think there should be any long term damage to the saddle.
