Ball moving after being stationary?

[SaintHacker]

Think I already know the answer to this but I'll ask anyway to make sure. Playing with my lad the other night, he hit a shot to the edge of the ditch in front of the green. The fairway slopes gently before getting steeper into the ditch. His ball was at rest on the gentle part of the slope. He walked up to his ball, had a look at his shot and chose his club, stood back and took a couple of practice swings, all the while the ball stayed there. However as he stepped towards it, and before he'd actually addressed it it rolled down into the hazard. Neither of us could understand how it happened' unless the wind gusted just enough to topple it forward. I would guess the ball was stationary for a good 2-3 minutes if you factor in the time for me to play my shot and then us to walk up to our balls. What is the correct ruling? Can it be replaced or is it now in the hazard?
Thanks

 

[rulefan]

If he did something to cause it to move then he gets a 1 stroke penalty and must replace it.

If he didn't, then it is in the hazard and he must play it as it lies or proceed under the water hazard rule.

 

[lyden]

If you agree he didn't cause it to move then it isn't at rest and must play it as it lies or proceed under water hazard rule.

 

[rulefan]

If you agree he didn't cause it to move then it isn't at rest and must play it as it lies or proceed under water hazard rule.

What do you mean by the words in red?