On my Rex HDE this has happened only 1 time, the tail hunting that is. I landed and found that the belt was shredding so I suggest you check that belt carefully. Spinning the rotor by hand revealed that the belt was moving side to side on the rear pulley and getting off the left side at times causing it to shred.

I installed the rear idler pulley from an MX400 and it solved the belt issue.

Other causes are the ball link on the servo horn is too far out from the center. It should be as close as possible with a digital servo. The gyro gain could be too high and the tail may be binding as well. Also make sure when you plug in the battery that you let the heli sit without touching until the gyro initializes if it's a heading hold gyro. Some details on your setup would help.