The most common cause is the way the floor was laid. Tiles set on a few dabs of adhesive, instead of a full bed, have air gaps underneath. Over time those dabs crack and the tile loses its support.
Heat plays a part too. Floors near large windows and balcony doors expand in the summer sun and shrink at night, which slowly breaks the bond. Water from failed grout or a leaking shower tray softens the adhesive from below.
If a hollow tile has already cracked, see cracked and broken tile replacement in Dubai. If it is loose and rocking, our loose tile reinstallation service re-beds it. Fine cracks in the glaze are covered under hairline crack filling.