Some natural stones and agglomerate resin tiles are moisture sensitive. So when thin-set mortar comes into contact with them during installation, water is absorbed into the bottom of the tile from the thin-set and it causes the tile to warp. The bottom of the tile expands, but the dry top of the tile does not. Just like a wet sponge on a kitchen counter top, as the top dries and the bottom of the sponge remains wet, the sponge will curl.
The solution? Use waterless epoxy adhesives that meet ANSI A118.3 to keep the stones from absorbing water and thus preventing warpage.
Epoxy adhesives are also necessary to use with natural stones that have fiberglass resin backs because they will bond to the resin back, whereas thin-set mortars normally do not bond well to resins backs.