The answer to 'why is my tree mud' is that your tree is likely suffering from a condition called 'muddy roots'. This happens when the soil around the tree's roots becomes oversaturated with water, causing the roots to become waterlogged and the soil to turn into a thick, muddy mess.
Common causes of muddy roots include excessive rainfall, poor drainage, or even a broken water pipe near the tree's base. If left untreated, muddy roots can lead to root rot, stunted growth, and even the death of the tree.