Ball moved by players or natural forces?

On the par 3, the player's tee shot landed in a difficult rough area right on the edge of the bunker. The player stood in the sand and took a few practice swings at a sufficient distance. He hit a few shots through the dense grass when suddenly his ball moved and rolled into the bunker. What had happened? And more importantly, what happens next?

A referee was called in to describe the situation and proceed according to the rules.
The referee had to clarify who caused the movement of the stationary ball.

If it was the player, Rule 9.4b would apply, which states:
"If the player picks up or deliberately touches his stationary ball or causes it to move, he incurs a penalty stroke."

However, if it were decided in favour of the player, Rule 9.3 would apply:
"If forces of nature (such as wind or water) cause the player's stationary ball to move, this is without penalty and the ball must be played from the new spot."

In this situation, the decision was made in favour of the player, why?
The referee had this situation described to him in detail:

"It wasn't deep", so the player said about the original position of the ball in the rough.
The practice swings were also made at a sufficient distance from the ball.

"The ball could have fallen at any time. He didn't move it. It was literally as if the last bit of grass had held the ball up." For the responsible referee, the situation was clear and the player was allowed to continue playing from the bunker without penalty in accordance with rule 9.3. He could breathe easy - especially as the situation from the sand was much easier than in the rough.

If you would like to know more about the rules of golf in the meantime, please contact me directly at hans.kienesberger@golf.at.