Cracked bathroom tile - runs almost entire length of the wall
I had a similar problem in my house, and after considerable whining from my SO, I started digging out and replacing the cracked tiles.  That turned out to be rather tedious, as the tiles were well attached to the wall and there was much collateral damage.
Ultimately my diagnosis was as Aloysius Defenestrate hints at in the comments.  The cracks were along a seam between two backer boards.  In my case concrete board below, green board above.  The boards where not obviously taped at the seams or supported by any horizontal lath boards between the studs.  Either one might have mitigated the problem.  Inevitably I ruined the underlying backerboard removing the cracked tiles, but I could see the underlying structure.
My repairs involved pre-drilling and toe-screwing some lath boards as support. and layering up new backer, thinset and tiles.