The issue of tiles popping up after several years is quite common. There are many theories behind this and one of the main reasons is the tiles are laid together too closely at a gap of 1mm or less. It is recommended to have at least 2mm of gap between the tiles, or 3mm. Due to the properties of teh tiles, sometimes the tiles may contract and expand with heat, and if the gap is too little, the tiles will push against each other which cause some of the portion to pop up.

The hollow sound when you knock on the tiles ie. "emptiness below tiles" is because the tiler uses very little cement to bond the tiles to the ground, either to save cost or to finish the tiling works quickly.