The action of pulling in the mainsheet (or kicker or whatever) can build up a twist in the rope, as your hand can turn slightly, every time you pull the rope.
To avoid this you can simply twist the free end of the rope a few times the opposite way, then sheet in & out to apply this untwisting to the rope. Additionally if you fit the mainsheet with only a 2x at the back of the boom and sheeting to the thwart (ie standard thwart sheeted mainsheet with a rear bridle), this will be less of an issue. With this set up, with the sheet going almost block to block on a beat, and 2x purchase, there is less potential for a twist in the purchase.
Dave8181