A post on this Blog, 15th Oct 2015, shows this fungus ----- and this is the result of fungal decay.
This oak tree looked healthy with a full leaf canopy last year and, other than the base, looks quite solid.
This species causes a 'butt-rot' affecting the buttress zone and principal roots. An intense white rot makes the wood brittle and prone to stem breakage or, as here, uprooting. This is why it is so dangerous when the affected tree is in an amenity area or along a road.
The next photos are the fungus at the base of this tree last year. The bottom picture is of an oak tree near Llandeilo which still stands although the fungus is far larger and brackets have appeared over several years. I am waiting for fresh fb's to form on this tree (to confirm my identification!). Few books show pictures at different stages of development. Although the specific name suggests this grows on Ash trees it can be found on several broadleaf species, Oak in particular.