Why would ceramic tile laid over concrete slab come up after a flood?

not the correct mortar used, or air gaps when flooring was laid and water seeped in.

WHOA !! Think this through before taking any action!!

Do Not go hiring an attorney or making a claim that the "Correct Mortar" was not used. That could lead to Months if not Years of litigation. Then you'll probably realize that no one will be able to Prove what happened and you've wasted a lot of time and money!

A tile install is an Art Form practiced by any number of people, some of which don't understand the Science behind a good install.

Mortar usually referred to as Thin Set is a cement based mixture of cement, fine sand as an aggregate and a pigment to the color of your choice. There are conditions that will compromise a mortar mix: too much or too little water, flash set, or holding the mixture too long before applying to the surfaces. This is the Art part. The installer needs to know what he's doing.

The Science part involves achieving a good bond. You need a good bond between the concrete floor and the thin set and a good bond between the underneath side of the tile and the thin set. By virtue of the tile manufacturing process; in almost all cases the tile does not absorb mix water from the mortar and a good bond is fairly easy to obtain. [Need to make certain there is nothing on the underneath side of a tile to keep the mortar from bonding].

The concrete slab is porous and will absorb water from your thin set! This causes essential water in the mortar mix to wick into the concrete and not allow for complete hydration of the mortar mix and a questionable [at best] bond is formed. This will happen unless you waterproof the concrete substrate Before you attempt a tile install.

A concrete slab that is not waterproofed will also expel chemicals from within the concrete slab that cause a dis-bond of the thin set. Alkali, efflorescence, saponification and alkali silica reactions in a concrete slab are all factors in breaking a bond. In your Flood, the concrete most assuredly absorbed Flood water and caused internal chemistry of your slab to come up to the surface that destroyed the bond. After you remove the tile and get an acceptable profile, Waterproof the concrete slab with a sealer that penetrates into the slab and fills the capillary voids with a solid material. Insist on a warranty for waterproofing by the product [15 years] and do not even consider a concrete sealer that allows for a re-application !! What happened to the first stuff we applied??

Obtaining a bond between cementitious materials is at best, tircky. It's impossible unless the host material is waterproofed!! That's the Science !