The Power of Reiteration
A fallen tree will frequently try to save itself by sprouting one or more new trunks. At first, these new trunks depend upon the survival of the old root system. Eventually, they may put down roots of their own.
A hollow may eventually be the source of the instability that causes a tree to fall, but roots may form inside the hollow. By the time the old tree is unstable, a new tree may have formed from the inside out.