It seems like this answer would be simple, but it is not.
It is not death that brings them down, its all the new life they create as they die.
While they are old and weakened other things move in.
Insects bore holes through the bark.
Birds peck at the holes to get the insects.
Water gets into the bark and fungi follows.
Now water hides in all those nooks and crannies.
Lichens like this environment so they start settle in and grow.
Moss spores get blown on the breeze and light on this new little habitat.
More rain, more spores, more insects, more holes.
One day there is a wind or a heavy rain.
There is not enough old dead fiber left in the bough to support all that new life,
so it breaks.
Gravity brings the bough back to earth with all the new life on board.
Death is part of a continuum.
Not the end.