The lack of a pan liner or surface membrane is usually the main reason these problems occur. Professional tile contractors who know the importance of a properly built shower with a pan liner & pre-slope, a bonded waterproofing sheet membrane or a liquid applied membrane.
Mold and rot occur when water gets absorbed into the grout and then seeps into the cementboard or drywall and then starts rotting the studs. Also when drywall is used the problems are even worse because the mold feeds on the paper face of the drywall. Mold loves cellulose and spreads throughout the damp wet area behind your wall.
As the leak goes unnoticed the damage and mold become worse.
When a shower liner is used without a pre-slope the water sits in the mud pan and slowly deteriorates/breaks down the mud bed and fills with mold/sludge. Sometimes the weep holes then become clogged with calcium deposits and efflorescence forms on the grout joints from the water drying up through the grout to the surface. A bonded surface membrane will solve these problems.
While we still do and have been doing traditional pan liners for as long back as I have been doing tile for (21 years), newer, updated ways of waterproofing a shower have been developed like Schluter Kerdi, Noble TS, Laticrete HydroBan Laticrete Hydro Barrier & Custom Building Products Redgard.
All excellent surface applied waterproofing membranes.
I tell customers it's good to spend the extra money now to do it right or you will spend 3 times the money tearing it out a few years later by doing it wrong.
