Thanks for your replies.
Yes it looks like it's been hit.
I'll check again today, I don't live at the property.
I don't know what condition the walls were in, but we had old tiles on there for years, no problems.
No pipes behind the walls.
It seems to be happening on all the walls with hairline cracks appearing (as in photo 1) more and more.
Our tiler used alot of adhesive, too much? Not dot and dab.
We'll have to take some off and see what's underneath.