Usually the trigger pin comes out, which allows the spring to fire the support pin out of the end.
Yup, that about sums it up. It happened while I was en route, so fat chance of finding the trigger pin. Fortunately, the support pin stayed in place until I removed it. It would probably be easy to molish a replacement trigger pin (I couldn't see a thread in the socket so I'm assuming it's the newer bash-in type) but I'm looking at this as a serendipitous excuse to buy a Classic rack, which I had been considering anyway. Two attractions of the Classic for me, as previously mentioned...
One, I've always been happy with the Bagman support except that because it mounts to the saddle rails, my saddle has to be set further back than I'd like. Using the Classic rack will allow me to set the saddle further forward.
Two, when off the bike, it provides a carry handle for the bag.