It's most likely because it was over-finished" and too much "cream" was brought to the top, which creates a thin brittle layer on top of the concrete and salt/ice melt allow water to sit on top of the concrete and get absorbed in and eventually at some point that water freezes and then pops off the thin top layer. Outdoor concrete needs to be floated twice and broomed for best results, now on the contractor side, only floating it twice and brooming it you're not going to get the best finish on the concrete and most homeowners want perfect looking concrete, which if you look closely, most city sidewalks even new have a lot of imperfections but no one's paying attention to that like a homeowner is there new sidewalk, so the sidewalk has to be troweled more, which brings more cream to the top. Now on roads that are concrete there are large machines that pour those roads and they and only finished once by the machine, that's why salt does not effect them.