The leaning Christmas tree is a common issue that can be caused by several factors. The most common cause is uneven soil or poor drainage, which can lead to root decay and the tree leaning away from the source of water. To fix this, you can dig up the tree and replant it on more solid ground, making sure to dig a deeper hole than the original planting hole and providing proper drainage. Additionally, you can use a stake to support the tree and prevent it from leaning. Make sure to securely tie the tree to the stake and check on it periodically to ensure it remains stable.