Not sure if this is the problem in our case, but I know from experience with several bipes that they all have what I call a "no man's land" right about the time that the tail comes up during a take-off roll (and, consequently, at the same speed during landing).

What happens, best I can tell, is that as the tail comes up, we lighten the load on the tailwheel...thus reducing its effectiveness in steering the airplane. However, due to the rather disrupted airflow at what is, essentially, a high AOA, the rudder has not yet become terribly effective as a steering mechanism. What's left is a plane that's almost entirely at the mercy of torque...and it tries to snap left. I've seen this happen in all 4 of the bipes I've owned, and, honestly, it's just something you have to get used to.