I suspect it might be the castor angle. The more angle, the straighter it will run hands off, but the steering will be slightly heavier. Lighter steering will give more kickback.
If you know what the spec should be - (from memory 2-4 degrees is common, but don't take that as gospel) - measure the backward lean of your kingpins. Yoy may be able to do this without fancy tackle - just by measuring the triangle and using schoolboy geometry.
