A common area for spalling concrete is underneath exterior formed concrete stairs. The spalling is caused by water seeping into the concrete corroding the rebar. As the rust forms on the rebar it expands causing the concrete to break off.
To prevent this from happening, you should have the top surface of the concrete stairs waterproofed. Waterproofing concrete involves prepping the concrete and then troweling on a special waterproofing cement. This will help prevent water from soaking into the concrete corroding the rebar.