Shower was redone about a year ago. At some point a crack slowly developed, through grout and a tile. The area is in the middle section, so tiles above and below and to the sides are fine.
What could be the cause? Is everything around it under strain, and it just erupted in the middle?
image below - red lines added over cracks
EDIT: another image with 3 steps - studs, cement board + redgard, start tiling
Also, I should say there are a few other grout cracks in various places. Bathroom is at corner of house, so it's an outside wall.
The reason for remodel was original drywall got wet and bulgy from soap holder falling off (and us not covering it up very well). Studs were a little moldy but seemed sturdy enough. Cleaned with vinegar and dried for a few days.