Cracked tiles in shower enclosure

[RrogerD]

My shower enclosure has been in place for about 11 yrs now. For the first 8yrs or so I'd had no issues. However, since then I've had problems with leaking and now, many of the tiles appear to be cracking (see the last pic with the crosses. These are the cracked tiles). The grout is also loose in places.
My question is: what is going on here?
The other pics show the construction and installation, taken in 2013 and 2014.
It is all built on the first floor, on timber joists. Stud-partition wall faced with Hardi-backer and 200 x 100 tiles. The tray is affixed with sealant on to chipboard on to braced joists. The shower enclosure has a sliding door. The entrance to the room, door is next to it.
I am thinking: banging of the sliding shower door? Banging of the main entrance door? Settlement / movement of the tray either on top of the joists or, the joists themselves?
Any other possibilities / suggestions gratefully received.
Thanks.

 

[terryplumb]

Expansion / contraction of the stud work coupled with a wall tile adhesive and grout that is the wrong type, ( not cementitious and flexible)

 

[RrogerD]

Expansion / contraction of the stud work coupled with a wall tile adhesive and grout that is the wrong type, ( not cementitious and flexible)

Ah, good call. Thank you. Hadn't thought of that.
Although.... it is all on hardi-backer. Presumably this does not expand and contract? Or at least as much as wood. I thought that it was supposed to be stable?

 

[terryplumb]

It is quite stable ,but the timber it is screwed to isn't. Virtually everything expands / contracts to some degree.Doesn't take much movement to crack tiles,that are stuck to unstable substrates particular when non flexible adhesive is used.