I recently took this picture of a slate windowsill near Broad and Morris Streets in South Philadelphia. You can see that water, cold, and pollution have taken their toll.
Slate is a stone that is created from layers of clay that have been compressed together over millions of years. The layers are easily split apart, which makes the stone easy to form into sheets for steps, paving stones, window sills, and shingles. Unfortunately, this layering is a potential source of its downfall.
If this particular piece of slate falls on the more porous end of the scale (which I suspect it does), then it's probably been absorbing water for a long time. Most of us know that water expands when it freezes. As the freezing takes place, it forces the layers of stone apart. We also have a lot of air pollution here, and porous stones can be weakened by acids in the environment, making the deterioration more rapid.
A good preventative measure would have been to apply a sealer to the stone to protect it from the elements. The sealer will need to be reapplied every few years in order to maintain it.
Remember--a diamond may be forever, but depending on your environment, your building stone probably isn't.