Completely done wrong!! First of all 1/4" should never be used over a standard sub floor. ( I assume thats what you have).. It doesn t meet tile standards. for minimum total substrate thickness.. 1 1/4" is the minimum for tile.. Thats the normal 3/4" sub floor and 1/2" cement board. Plus it needs to be attached ( nails or screws) every 6 to 8" and closer on the seams.

Plus a thin set must be used to take up any "air pockets" or space under the cement board..

Couple other possible causes include wrong size trowel. to wet or to dry of thin set causein loose tile.

Also possble is to wet or to dry grout when installed causing a weak grout. Any or all are possible causes.

He/she needs to return and do it right.

Any questions you can e mail me through my avatar and check my qualifications there. GL

2 or 3 yrs ago I had the hall bath front entry floored with 1 ft tile over cement board. Looks great! Love it!

Last month, I had a different contractor use the same materials in my main bath and back entry...and the grout is crumbling! Yes, I want to cry. I noticed that he only screwed the cement board around the edges. I hope this is not causing too much flexing over the OSB. I think the cement board was only 1/4" thick?

Mostly, it's just been me to walk on it. I don't think anyone over 180# has walked on it. I'm hoping it's just the grout which was mixed incorrectly. Yes, I'm calling the contractor. *sigh*