Ian, you've refinded that idea nicely. I'll have to check the possibility of up and down motion. When I had the rudder off (lets say three years ago) I had to fudge with the adjustment of the mounting nut to allow the rudder to swing freely and the the hole for the pin was clear, so there may be enough play to allow the motion you describe. V's rudder is not wood and it is some heavy, but that doesn't mean it couldn't have motion (or intertia) that is counter to the hull once in a while. Thanks for the fresh lead. I wish I knew if my pintles were original or just whopping great bolts that replaced originals. Mine has the castellated nut, but I'm trying to remember if the thread went all the way up the shaft of the pintle or not. I think they do, and there's a hex head top to it a la bolt. Are yours like that? In the mean time, it doesn't happen often and if the hardware isn't broken I shall, perhaps, resist fixing it - Securing the rudder head instead of just the tiller might cure it.
On a slightly related topic, I completely re-worked the hardware assembly that makes up V's gooseneck when I realized that when the main was reefed, all the tension of the main sail on the reefing horns was being held down by nothing but a light cotter pin! And that cotter pin was, indeed, chewing into the base of the boom end casting. I had my tallented friend make a new gooseneck pin for me out of a S.S. bolt whose only job now is to hold the boom to the mast fitting. I did away with the reefing horns and put a S.S. snap on either side of the now beefier shackle that holds the tack of the main to its fitting on the boom, and it's now those snaps that take the vertical load of the halyard when reefed.
Note added later as a result of later posts: The new gooseneck pin was made from a long carriage bolt. The treaded part was cut off and a hole drilled for a cotter pin.