I very much doubt it's a structural issue. Youd see other issues to accompany it so I reckon you can put that out of your mind safely
Thermal shock due to the temperature change would probably do it especially if its underfloor heating. Specially if the heating was turned on at a high temperature relative to the ambient temperature of the slab. When turning on the heating aftercare cold spell its always a good idea to treat it as if it's the first time and turn it on slowly. Given there was some moisture that might also have played a part in that the adhesive or primer may have been weakened due to dampness. It's possible that there are not sufficient movement joints in the tile face