A few more things that can mess up a tile install: letting the thinset skin over (i.e., let it sit too long - spreading too much at one time) before setting the tile; walking on the tile too soon after setting; poor thinset coverage; a kiln release film on the tile that wasn't cleaned or thinset not 'burned' into the back; dirty tile; grouting tight up to the wall or baseboard (needs a gap at the room edge).
But, the biggest one is probably inadequate prep of the structure. As already mentioned, if there is a failure there (subfloor not strong enough, joists too bouncy, improperly attached subfloor), it can cause failures. While it is possible to install tile directly on a properly prepared plywood floor (requires two layers, installed in a very specific manner), most people use a cement board (cbu). If the installer did not embed the cbu in thinset or tape the seams, that can cause failures, too. Yes, you need thinset both under and over the cbu. The thinset underneath isn't to hold it down (the screws or nails do that), but to fill in any imperfections to give 100% support. Neglecting that, and you will have problems.
Another, unfortuneately common, problem is what is sometimes referred to as a Jersey mud job. This is where they install lath on the floor, then spread thinset on top and set the tile. This is not an approved method and has been shown to be unreliable, but some people continue to insist on using it. It rarely lasts more than a few years, and can fail quickly.
Lots of ways to mess up. There is more than one approved method to ensure a reliable install, but skip a step or poor workmanship can doom an install quickly.
So, some details on how they prepared the floor for tile might point to why it failed.
If you take something like a golf ball, or a baseball bat and tap the tiles, they should sound solid. If any are loose, they'll make a different sound and sound hollow. You're likley looking at a tearout and redo. More info would be needed to confirm, though.