It's the angle of the road, rising behind the deer, plus the all white surroundings which remove the usual clues for perspective, and the fact that the deer are compacted, lying, rather than the fuller forms we'd see when they are standing.
They have hollowed out the snow to block some of the wind, which also makes them look flatter, because they're actually lying in concave holes. Having seen lying deer in other settings, this doesn't look too "off."
Look at it a bit with those thoughts in mind and maybe they will look more natural.
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