It can be many things. Rudder servo could be weak , linkages could be loose , pull pull wires could have slack , rudder could be glitching (flopping with a mind of it's own) , could be the wind , controls could be too touchy (may need more expo)
Or it could be aerodynamics related... like a too small vertical stabilizer, or an airframe operating outside of it's designed flight envelope. For example, the Kaos is notorious for wagging it's tail when flown to fast. Could be propwash, tip wingtip vorticies, turbulence from the canopy, etc...