The answer to 'why is my tree mud' is that your tree is likely suffering from a condition called 'tree leaching' or 'tree bleeding.' This occurs when the tree's sap or internal fluids leak out and create a mud-like substance around the base of the tree. Common causes include damage to the tree's bark or root system, disease, or environmental stress.
The symptoms of tree leaching include a wet, muddy area around the base of the tree, discolored or wilting leaves, and potentially a foul odor. If left untreated, this can lead to further damage and even the death of the tree.