Also known as Flapback, caused by the difference in Ralative Air Flow between the advancing blade (getting more RAF) and the retreating blade (getting less lift). The difference is greatest at the 90/270 degree position, and the differences in lift make the advancing blade want to flap up and the retreating blade flap down. The dreaded "flapping to equality."
If unchecked, the nose would rise, the airspeed drop off to zero, and the aircraft then fly backwards, because that's where the disc is pointed now. The same thing happens backwards, so the tail flaps up, nose goes down, and it all starts again. However, the pendulum effect of the fuselage makes the nose kick higher at the front and then the nose kick lower at the back, so the oscillations increase each cycle until after about 3 or 4 cycles, you have crashed.
Simple fix - as you move forward and the nose wants to rise, add a little more cyclic to hold the attitude where you want it.