Totally random mmm, now that is an odd one. Need to know a few more things.
Has the underfloor heating been ran since it was tiled, has this been ran for a while, and when it was first switched on was it brought to temperature slowly or did whoever switched it on just let t heat up to full temperature.
If the underfloor heating has been switched on and allowed to heat to quickly then they may have been to much expansion in the floor for the tiles to take, but:
Maybe you havn't even switched in on yet.
Also if there is random cracks then this to me sounds like two things. it could be one of two possible thing:
Poor fixing, if the floor underneath is cracking it usually cracks in straight lines through several tiles. So random cracks would indicate that the tiles with the cracks havn't been bedded properly, and are breaking, the tiler may have 'dot and dabbed the tiles" lets hope not.
Or you say this floor hasn't been walked on, it may be the tiles themselves have cracks within them, bear in mind this is a natural product and stone does have veins that form a weak spot. These can show up after grouting, but really the tile shouldn't have fixed them.
Its very hard to say here without guessing, so can you send us some photos as at the moment we are only speculating.
Cheers.