A shower is designed not to act as a swimming pool but rather more like a funnel, directing water & contaminates down and into the waste drain at the base.

Each time a shower is used a very small percentage of moisture can escape the surface moving into the subsurface, where it should dry out between showers causing no harm and actually helping to keep the grouted area waterproof.

When a "leak" occurs (not in the plumbing) excess moisture is allowed to escape, this goes into the sub-surface and does NOT dry out between showers.

A leak can occur through cracked, missing or eroded grout, cracked or broken tiles or the seals where the walls  and the wall/floor meet.

More often than not it is the seals where the walls & floor join.

When a leak occurs excess moisture can saturate the floor then through capillary action it can rise into walls behind the tiles.

These leaks can be seen as: Damage to adjoining walls -looks like salt damp or paint peeling off. Wet carpets or floors in adjoining rooms. Damaged or swollen architraves &/or skirting.  Tiles cracked, broken, loose or discoloured. Persistent mould

My Theory

This is only my personal theory and I must point out that there has been no researched data or testing done to prove (or disprove) this theory.

On the other hand commonsense will show that my theory is not without merit.

New showers built since the new code (approx 1994) require a waterproof membrane and a puddle flange to the wet area, this has created a new problem.

The moisture that escapes the surface naturally, escapes (as described above) and can no longer get into the sub surface where drying can occur. 

This water is meant to drain into the waste system via the puddle flange but for a number of reasons (including: incorrect instalment and debris blockages) this may not happen. Instead it lies between the membrane and the underside of the tiles. if minimal moisture escapes this can dry out between showers  but anything that can not dry out remains, and with each shower increases. This moisture is affecting the back of the tiles (which is not waterproof only the glazed surface is) and the adhesive both of which can cause swelling, movement, cracks, loose tiles, mould etc. 

I also believe: if the water-proofing was done AFTER the screed was in place any water that escaped the surface would then follow the slope created by the screed towards the puddle flange and into the drain/waste .  At the moment the water-proofing is done to an often uneven base or floor.

These can all be the signs in an older shower but they can appear much earlier in newer showers. 

In some cases with the moisture/water trapped between the membrane and the finished surface it can find a weak spot in the membrane and escape or comes out near the lowest part of the membrane, that is, at the entrance (doorway) to the bathroom. 

Often the shower alone has a waterproof membrane (floor and corners) and not the entire floor or walls.

That is why our  process works by Stopping the water getting past the surface, not dealing with it once it has as is at present.