Kingpin angle, tire air pressure, spring tension, amount of grease, etc......the tailwheel by itself is designed to track straight without any steering inputs so having undamped springs pulling on it when it's already straight doesn't make any sense to me. I like the Maule springs because they aren't doing anything if the tailwheel is straight, it's free to track straight as it's designed to do. So I wouldn't say it has nothing to do with the springs.
From another angle, I also suspect we induce shimmy with too much back stick too early in the landing rollout when the elevators are still very effective. We're probably pushing down hard enough to change the kingpin geometry and inducing shimmy. I've seen where it goes away if I move the stick forward. It's hard to say don't bring the stick back after landing because sometimes you need it to stabilize the plane on the wheels but, combined with a high landing speed, it's contributing to shimmy, IMHO.
Just thinking some more, has anyone ever compared their two springs to see if they are exactly the same length and if they have exactly the same spring constant? When the tailwheel is straight, the distance from the control horns on the rudder to the steering arms on the tailwheel should be the same. If the springs aren't matched, then one side will be pulling harder than the other which could induce shimmy. I'd be curious to test a set of springs from a plane that has had a history of bad shimmy issues to see if the springs are way off or not part of the problem.
fyi, spring constant is the force needed per inch of stretch. If you have two of the same springs, hanging a certain weight on each of them should cause them to stretch the same amount.
