A common cause of failure is rusting iron fixings - although they were usually set into poured lead, the slightest gap will eventually allow water to penetrate. Rust can expand to nearly seven times the volume of the original iron, bursting stone with slow but inexorable force.
This sandstone cross was eventually dislodged in a terrible storm but fortunately landed on grass. It was simple to make a fresh copy and pin it in place with a long stainless steel dowel set in two-pack resin adhesive.
restoration