Originally Posted by
b0gy
I think you are right. I detached the cable from the rear derailleur and took out the housing and the back stop and there is some fraying there. Inside the lever everything looks ok. I will replace the cable and see if the problem gets fixed. Thank you Andrew!
You might need to replace the housing as well. But I agree that the most likely situation is that when you shift and there is too much cable friction, the cable goes slack and that allows the cable end button to pop out of its anchor point.