I was wondering about that too. If you believe the internet: "In baseball, a runner is not required to retouch first base between pitches if the ball is alive. However, they are required to retouch first base when the ball is dead, such as after a foul ball, a timeout, or if they are forced to return to their original base due to a play. Some runners may retouch the base between pitches as a habit, to measure their lead, or to reorient themselves, but it's not a required action."