Your problem is extremely common, so don't worry about it!

It's caused by one or both of two things;-

Your vertical speed is too great, and you have simply bounced, or your airspeed is too great and when the 'plane settles back onto its tailwheel, there is sufficient lift created from the sudden increase in wing incidence that it tries to take off again.

For the latter, you could try extending the length of the tailwheel wire, so that the three-point contact doesn't create as much upward angle of the plane, and therefore the incidence isn't so great.

Just a thought!

-David C.