I don’t see an obvious cause but here’s some general young tree care advice:

Ensure the root flare (a swollen area at the base of the trunk where the first large roots fuse together) is not covered by soil. Do not assume that the soil level in the pot was the correct level for it to be. Cheap nurseries that mass-produce trees often pot them too deep which leads to permanent health problems unless corrected.

Remove the grass around the base of the tree and replace with several inches of wood chips, but avoid covering the root flare. The chips should cover the soil around the tree but not touch the tree itself.

Check soil moisture regularly, and slowly but deeply irrigate during dry weather to keep consistent below-ground moisture. Allow the soil surface to dry somewhat before watering again. Proper soil moisture should be soft and moist but not sticky or dripping. Never allow the soil in the root zone (below the surface) to get completely dry. In future years the tree will be tolerant of dry conditions but not during year 1.

Avoid the use of lawn equipment, especially string trimmers, near the tree. Young tree bark is very delicate and can be easily damaged which can cause major issues if serious damage is done. Hand weeding close to the trunk is best. If the mowing in this area is outside of your control, consider protecting the trunk with a trunk guard. In a pinch, a plastic bottle with the bottom removed and cut down the side can suffice.