Stone was detaching all over the home foundation… and at the chimney too. That *tall *chimney. In most areas stones pulled loose and left the mortar on the block, or in the case of the foundation wall, on the concrete. The back side was an exception and a whole lower section had sheared off the concrete foundation wall taking mortar with it.
Over much of the work there was little or no mortar between stones, which were simply adhered to it (or not).
I’m thinking mortar froze before it set up enough to withstand freezing temps. Qualified masonry contractor recommended.