Deformation in selective laser sintering

[lingdeer]

I am wondering what causes warping in laser sintering? Can someone explain the mechanism to me?
For a single layer, which direction will it warp in? (towards or away from heat source), and how big is the deflection is it usually?

 

[Baluncore]

I would expect the sinter material to momentarily melt, then to solidify.
As the solid cools the differential contraction introduces distortion.