Whether a golf ball is sitting in tall grass, pine straw, leaves or some other form of underbrush, you have to be careful when you address the ball or you might risk accidentally causing it to move.
There have been numerous examples of this rules violation on the professional tours over the years, and rules makers decided to revamp what happens when a ball moves before or during a stroke back in 2019. For example, a ball that accidentally moves on the putting green can be replaced without penalty (Rule 13.1d). It doesn't matter if you did it or if something like wind caused its movement. The putting-green-rule update is generally regarded as one of the best the USGA and R&A made, as golfers used to be penalized if they addressed a ball on the green and it then moved. Windy rounds on slick greens were a lot of fun!
Back to the subject of a ball moving before you make a stroke, the Rules of Golf now says under Rule 9.2b that there are only four possible causes and you have to determine which one it was before proceeding.
(1) Natural forces such as wind or water.
(2) The player or his or her caddie's actions.
(3) The opponent or his or her caddie's actions in match play.
(4) An outside influence including any other player in stroke play.
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Let's set aside Nos. 3 and 4 and just deal with whether you caused the ball to move or if natural forces made it happen. If you take your club back and your ball sinks farther down into the rough, it has to be "known or virtually certain" that you caused the ball to move for you to be penalized under Rule 9.4 (one-stroke penalty). To refresh your memory, known or virtually certain means you have to be at least 95-percent certain.
If you're not virtually certain or there is no definitive evidence, the ball is treated as being moved by natural forces. Now here's an important thing to remember: regardless of what caused the ball to move, how your proceed depends on what has taken place. If natural forces move the ball, play it from its new location. But if you moved the ball, in many cases (but not all), you have to put the ball back where it first came to rest.
For example, tour pro Ben Kohles lost his chance at a PGA Tour card at Q School in December when he moved loose impediments around his golf ball and it caused the ball to move (resulting in a costly penalty). Because he had yet to play his shot, he was required to replace the ball in its original spot. That's what you should do, too, otherwise you'd also be penalized for playing from the wrong place (loss of hole in match play or a two-shot penalty in stroke play).
Even if you intentionally stop your backswing or downswing because you see the ball sink down in the rough, you still have to replace it. The exception to this would be if you actually made the stroke anyway. Then you play it from wherever it ended up.