+1 Expo can be what's causing the problem.. on a taildragger the rudder can be very sensitive during the takeoff rollout...
if the issue is that you're over correcting, as has been mentioned, try using either a low rate setting for takeoff or dial the rudder down.
it takes a little getting used to (and some airplanes are more sensitive than others) but you'll get a feel for exactly how much rudder to apply and when.
(just breathe on it
)