I've seen multiple attempts, trying to explain it:
Because of the formulas for momentum and kinetic energy, only the same number of balls can fly of at the other side. (found like this in many textbooks)
The first ball hits the second ball, the second the third…(often explained with billard balls) If multiple balls are lifted at the start, it works the same, but two times directly after each other
At the point of collision, two Shockwaves start to travel in both directions, reflect of the respective end of the cradle, and the cradle separates at the point they meet again.
So which one is correct?