Just a guess on why they turn to stone, the Maiden Arien from among the Maiar is who the Valar chose to guide the Sun.Servants of Meklor could not endure her glance it was said so maybe Trolls of all of Meklors fellbeasts if indeed they where of his making could endure it the least and would return to their natural form (stone).Another guess would be they are just beasts from before the lighting of the Trees of Valar and just happen to not be able to stand light for whatever reason but i dont think Tolkien ever really said why.
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He who arises in Might.
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