I've always felt that that vision of Merry's was connected with the jewelry that he wore, and looking at the passage more closely, I noticed the mention of
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...the golden circlet that had slipped over one eye.
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Did that cause the 'slip' in his sight? When he comes to himself again, it is with:
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'No, no!' he said, opening his eyes.
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(bolding mine in both quotes)
In short, the jewelry was affected by being in the Barrow, so it had some magical properties, I think. When Tom takes it out and lays it on the grass, it breaks the spell.
But the question remains, why Merry, why not Pippin (who was more often the one with a sense of supernatural 'sight')? Sam I can understand - his prosaic nature gave him the ability to sleep ('in deep content, if logs are contented') in Tom's house while the others dreamt.