[pbhj]

If Denise moves the ball to the bucket then perhaps putting balls in buckets is more common than putting balls in other containers. Sally being a child is likely to mirror Denises behaviours and therefore is probably more likely to place the ball in the bucket herself. She is thus more likely to look there than in other containers in which she herself would not place the ball.

If one knows that the ball is in the bucket this reinforces the concept that balls go in buckets or reminds one that more often than not people put balls in buckets.

Similar to the way that lots of people pick the same number in a given range or pick the same fruit when asked, etc., the most likely container for people to pick may be a bucket. People could be responding to an innate understanding of this fact; possibly without realising it.

Speculation, but still a decent logical explanation IMO.

A pre-experiment should be performed by asking people which vessel a ball should be placed in; the experiment in question could then choose only vessels with matching preference as ball receptacles as the options.