Where the crack is tells you the cause
Cracks in the internal corners, or along the line where the wall meets the tray, are movement joints doing what grout cannot. Those junctions should have been sealed with silicone, not grouted, because two surfaces meeting at an angle always move relative to each other. Grout in a corner will crack every time, no matter how well it was mixed.
Cracks running across the middle of a tiled wall, following the tile lines, point at movement in the substrate behind, usually a flexing board or a wall that was not rigid enough. Random crumbling across a whole area with no pattern usually means the grout mix itself was wrong.
Bad mix, fast fix, short life
Grout mixed too wet is the classic. Extra water makes it easy to spread and leaves it porous and weak once cured, so it powders under a brush within a year or two. Grout mixed in small batches with inconsistent water gives you patches that behave differently along the same wall, which is why one section is rock hard and another crumbles.
The other common shortcut is grouting before the adhesive has cured. Trapped solvent and moisture make the joint weak, and on large format porcelain, where the adhesive bed is thick, that curing takes longer than people expect.
The substrate problem you cannot see
Tiles fixed to standard plasterboard in a shower are living on borrowed time. Once water gets through the grout, and it will, the board softens, the tiles flex a fraction under the weight of somebody leaning on the wall, and every joint in that area starts to crack. The same happens on timber stud walls that were never noggined properly.
On a shower fitted this way, re-grouting is genuinely a waste of money. The correct repair is to strip back to the studs, fit a waterproof tile backer board, tank the wet zone and re-tile. It is a bigger job and we will always say so up front rather than after starting.
Repairs you can do yourself
If the cracking is limited to one small area and the tiles are firmly stuck, a local repair is realistic. Rake the failed grout out with a grout rake or an old flat blade screwdriver to a depth of at least two to three millimetres, vacuum the dust out, wet the joint slightly, then work new grout in with a rubber float pressed hard across the joint rather than along it. Sponge off cleanly and polish the haze once it has gone matt.
Where corners are involved, do not grout them. Rake them out and seal them with sanitary grade silicone instead. That single change stops the crack returning in most bathrooms.
When to have it looked at
Get it checked if grout is failing across a wide area, if the tiles behind it sound hollow, if the grout stays dark long after the shower has been off, or if there is any damp showing in the room next door or below. Those all suggest water has already found a route past the tiles.
We re-grout and re-seal as a standalone job, and we will tell you honestly when a re-grout is the right answer and when it is only cosmetic. Send a close photo of the worst joint through the enquiry form for a straight opinion.