Opinion on Loose Floor Tiles

Im inspecting a small 1600 ft2 home on a slab foundation. About 90% of the ceramic floor tile in the kitchen, entryway and restrooms are loose, or they sound hallow when I tap on them. The tiles are not cracked, but the grout lines are. Any ideas of the cause? Could this just be improper installation? There are no signs of severe foundation settlement. Any thoughts or opinions would be great.

This is a duplicate thread.