Answer: The most common reason for a tree to lean as it grows is that it is reaching out for more light. It will grow toward the light. If the neighbor's plant is shading your tree, then that might cause it to lean. If the house or houses shade it, then that could also cause it to lean.
If the shade is caused by the neighbor's plant, you could talk to the neighbor about trimming it back. If the shade is caused by the building(s) then you might consider moving the plant so it will be in full sun all day long. Since it grows so large eventually, you want to make sure it has ample room for its mature size anyway -- for Acer platanoides "Crimson King" allow about forty feet tall and forty feet wide plus extensive roots spreading wider than that. If you think it needs to be moved, do that in late fall right after the leaves drop or in very early spring as soon as the soil can be worked.
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