movie - Why do the trolls turn to stone in daylight?

I've been watching The Hobbit Extended Edition, and saw the scene where the dawn light inundated the three trolls, who promptly turned to stone. What I have to wonder is, why do the trolls turn to stone when exposed to sunlight? I'm aware that not all do so, but why these ones? Is it something to do with their origins or nature?

Answer

Tolkien Letter 153 implies that since Trolls were created from stone by Morgoth using a spell cast in darkness, they revert to their true nature when exposed to the sun (emphasis mine):

I think [Trolls] are mere 'counterfeits', and hence (though here I am of course only using elements of old barbarous mythmaking that had no 'aware' metaphysic) they return to mere stone images when not in the dark.

This is also spelled in Tolkien, An Illustrated Encyclopedia:

The spell of their creation had been cast in darkness and if light did fall on them it was as if the spell was broken and the armour of their skin grew inwards

It is important to note that, in Tolkien's world, the Sun is in fact guided by a Maia named Arien, who is one of the few fire spirits not corrupted into Balrogs. Morgoth himself feared her regard, hence the perpetual shadows used by Sauron and him.

And Arien Morgoth feared with a great fear, but dared not come nigh her, having indeed no longer the power; for as he grew in malice, and sent forth from himself the evil that he conceived in lies and creatures of wickedness, his might passed into them and was dispersed, and he himself became ever more bound to the earth, unwilling to issue from his dark strongholds.. With shadows he hid himself and his servants from Arien, the glance of whose eyes they could not long endure; and the lands near his dwelling were shrouded in fumes and great clouds.

They were also at least two kinds of Trolls: the ones in The Hobbit were stone-trolls, but Sauron latter created the Olog-ai using a upgraded spell, which did not petrify when exposed to the Sun.

Unlike the older race of the Twilight they could endure the Sun, so long as the will of Sauron held sway over them.