A bloke I know lives in sheltered housing that was built about ten years ago and the other day I noticed in his bathroom that a couple of the tiles on the wall were protruding slightly as shown on this photo (which shows the tiles viewed from below).

https://www.dropbox.com/s/66a3v0gvii1boon/tiles.jpg

On closer inspection I concluded that the all of the tiles on that wall, about 4 square metres, are loose and coming away from the wall, all that’s holding them together is the grouting. I can push in the centre and the whole lot move!

I can’t think what would cause this to happen. I thought maybe there’d been a water leak from the bathroom of the flat above that has come down behind the tiles and gradually washed away the adhesive but there are no signs of dampness anywhere in the bathroom or in the room behind the stud wall where the tiles are.

Anyone got any ideas what might cause this?