Grout Isn't Forever
Even in the places designed to withstand moist and humid conditions, a tiled surface is not waterproof. The tile itself may not allow moisture to penetrate, but the grout, or thin lines of mortar between the tiles, is susceptible to water damage. If you leave water standing on the surface, it can loosen the seal between the tile and the grout, allowing water to leak through. 
Cracks and Gaps
Cracks in the tile, gaps in between the tile, or chips in the grout can allow water to make its way underneath or behind the tiled surface. The crack does not need to be big for standing or excessive water to create considerable damage. Repair any tile or grout that shows damage or excessive wear to prevent water from making its way below the surface.
Behind It All
Once water makes its way past the tile or grout, it starts to do real damage, including loosening the bond between the tile and the subsurface. The mortar that holds the tile in place will start to loosen and crumble and the tile will fall away. In addition, if the subsurface is not waterproof, it will swell and warp, further pushing the tile away. Mold and mildew may form, which eats away the mortar even further.
Damage Control
Regular maintenance and limiting water contact can prevent water damage to tile surfaces. Do not allow standing water on tile surfaces whenever possible. In cases where you can't avoid it, remove it as soon as possible. Use a squeegee or mop to wipe away puddled water.