Root Fungi in the Fall

This is the time of year when I see the clumps of fungi appear in my yard. Seemingly overnight a clump will appear. Light brown in color and consisting of dozens of stereotypical capped "mushrooms." They almost always seem to be associated with surface tree roots. My trees are old and deteriorating. Does anyone know if this is an invasive fungal disease of trees or merely a sign that roots are dying of other causes?

Thanks