We had the exact same thing happen in our old house. It turned out there were two issues at play. The floor wasn't actually leveled properly and there was dust or some kind of debris that was around and created issues. It was a huge headache.

It looks like that tile didn’t have much contact with the adhesive.

Properly installed tile would never come up that easily and cleanly ...

What I have observed is that the tiles closer to the walls seem to go hollow versus the center areas of high traffic didn’t go hollow until 1-2 years later. There are still several that are still adhered.

I don’t really see a reason for the installer or builder to hide what may have gone wrong but it’s nice to get other feedback as well now that things are visible.

The biggest thing I want to address is whether it’s a tile issue so that the tile is not reused and then we have an issue again.

Left side - installed tile I pulled up

Right side - stored extra tile

Interesting that the color is that different unless it’s simply due to a different lot.

When I tiled my floor the thinset we used was way more grainy and cementitious than that looks. Are tilers using premixed mastic for floors? I could totally see that skimming over and not adhering. We didn't back butter because the tile size didn't call for it. (We've had no issues and there nothing that sounds hollow...)

"I don’t really see a reason for the installer or builder to hide what may have gone wrong"

You're too trusting. He already knows why the tiles are popping up.

The installer let the mortar sit too long on the floor or in the bucket before pushing the tile in, it either skinned over or started to "go off" before the tile was pushed on.

While yes this isn't the best troweling job ever, you can see that the tile surface did push the diamond pattern in and push down the tops of the trowel ridges over nearly the entire area. There should have been enough contact to hold the tile down, but the mortar did not grab the tile where it made contact.

This is why I always back-butter. If there had been fresh mortar on the back of each tile as laid down it might still have been okay.

Installer Error https://youtu.be/Way5bMh-eYg

Thats an excellent video.

So the mortar could have skinned but it looks like we have the following errors - at least for this tile.

1) Not troweled in one direction

2) The ridges weren't collaped

3) The troweled direction should have been in the direction of the short dim of the tile

Makes me a little uneasy that I have 3 other decent sized bathrooms including seperate tub and shower. Although I specifically remember seeing a different "helper" guy do the lauddry / mudroom which is what I have the issue with. The owner did everything else from what I recall.

This is a redo. Plain and simple. A tile floor is a forever floor. It should not move until you jack it out with hammer and chisel.

Your floor has failed and is continuing to fail. I'm not worried about the tile...I'm worried about the tile SETTER.

I would get this in writing.