That seems like the most plausible reason.

Tiling over old tiles isn’t that uncommon, and here I suspect they used very little glue (or poor quality glue) so as to not increase the thickness of the floor too much (to avoid issues with thresholds etc.). Thin and/or low quality tiles could be another explanation, as to why they crack lack this…

Either way, I can’t think of a way to fix this without ripping up the old tiles and laying new ones.