Could be a couple of things, most all of them have been mentioned here:
1 - Momentum from the flight before touching down, or coming in to land at an agressive angle to the runway. Come down easy and straight, imediatley after touching down pull your flaps all the way up and then apply the brakes a second or two after letting it settle down on the runway a little bit. Definetley get your tailwheel down and locked on the ground ASAP to help stabalize the plane while it's on the ground. Learn the left and right wheele brakes too, they ontop of the rudder come in handy for lightly tapping to correct any directional problem you might have on the ground.
2 - Your trim settings are likely set in automatic combat mode. This means the trim is settup to counter the torque and p-factor of your aircraft, with those factors gone or throttled down then the trim is causing you to drift in teh oposite direction. Learn to manual trim a bit in the game if you haven't already. It'll come in really handy when you need it (IE: If you loose a big chunk of your plane and full stick in __ direction just isn't enough input to counter it, full trim might just be that little extra input you need to make it home. You go into a fast dive and your control surfaces become unresponcive, start slaming your trim in the desired direction you need to go to get out of the dive and the plane begins responding again quickly. If you have an engine problem and you need those extra yards/miles of efficient engine-less glideing to make it).
(3 - I'm assuming the landing strip you're trying to tackle is not moving. If it's a carrier, there's always the possibility of it turning and changing cource on you while you land.)