I do see that the cracks are above the seam in the subfloor Ok that makes sense to me. Even though from underneath you can't see movement when someone walks above, that doesn't mean there isn't a little give. It doesn't take much movement at all to cause tile to crack.
Perhaps someone heavier walking above would do it.
Normally the seams in your subfloor would be right on top of a floor joist, unless it's been double decked. Double decking doesn't matter where the seam is on the final layer.
Lets assume you have a single layer of decking.
The seam will be right on top of a floor joist.
Locate that floor joist from underneath and install some bracing across to the floor joists that are on each side of that one.
Lets say your joist are 2 by 10's and they are 16 inches apart.
You could buy a 2 by 10 and measure what the space is ( should be about 14 1/2 inch for 16 inch center ) cut 4-6 boards at the proper length, ( when you have to tap the board in with a hammer to get it to go, you are about right ). If you put in about 3 on each side about a foot apart, that should fix it. Also use 3 inch long wood screws to secure the bracing.
What you are doing by doing this is strengthening that one floor joist by making the other two help share the load.
Hope this makes sense to you, it's kinda hard to explain.
Without being there to see for myself I can almost guaranty you that, that one joist has just enough give to cause this.
Count
As you slide down the banister of life, may
the splinters never point the wrong way...
