Very strange. This is usually due to a lack of nitrogen and perhaps something restricting the roots. Sometimes a planting hole ends up being dug with slick sides that restrict roots and work like an underground flower pot with circling roots. There is not a good way to fix this other than digging down vertically to cut roots and break up the edge of the hole. Occasionally this is due to a minor amount of damage from a weed and feed product or other herbicide. Time is the only solution to this.
I would give it more time and hopefully it will pull out of this. In the meantime dig around the base about 6" deep to see if a root or other object is strangling the trunk.
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