Plane go crazy while i give descending command in RTL mode

Hello. I have strange behavior.
Usually, if you raise the pitch stick up during RTL, the plane will descend smoothly. But this time something strange happened. At first the plane increased its speed, then began to sharply descend with a flip. Switching to FBVA saved me from a crash. The plane leveled off in this mode. I did not repeat this operation again this time.