That is what I was originally thinking and then got convinced by The Great Marquis' explanation - making my next unit of cavalry has come to a halt, not sure I wanted to rip off and reposition all holsters that I had already finished.
VIRTUTIS GLORIA MERCES -- Glory is the reward of valour.
It does seem to make sense doesn't it, I wouldn't bother removing holsters the 'wrong way round' - I'm working on the assumption that the pistols were carefully placed in holsters for ease of access and rolled around in them as soon as the trooper took his eyes off them, Basically a C17th demonstration of Sod's Law!
I love the notion of the trooper riding around in a cloud of loose equipment! Have a look at some contemporary pistols and/or holsters. Alternatively, have a look at Wagner page 74 which shows how the pistols were carried and drawn. The text reads:"Pistols were kept in holsters with grips pointing forwards to ease grasping and drawing".Page 249 of the same book has a drawing showing the relative sizes of 17th century weapons relative to a man. It should give a realistic idea of just how big a horse pistol was. Better still, there are some in the Royal Armouries in Leeds that I think came from Littlecote House and were the arms and equipment of Popham's Horse during the Civil War. Plate 4 of my copy of Haythornthwaite shows a cuirassier holding a pistol which also gives an impression of the sheer size of the pistol.It's not a case of what take's the rider's fancy, its the physical need to draw a pistol that long from a holster long enough to contain it. If it's physically possible to raise the arm that high at all it is much more awkward than drawing the pistol. like a sword from left to right across the horse's neck (and vice versa).Coming back to my opening comment I think it is unlikely that holsters that do not hold the pistol in place tightly would be welcomed by any cavalryman (even a treasonous Roundhead)! A brisk trot or canter, let alone a gallop, and the pistol is likely to fall out. I can't off hand think of a holster from any period that will hold a pistol the wrong way round.