From the official FIA release that you can read here:
https://www.fia.com/file/72438/download?token=tISsFCi-
Renault submitted that Art. 3.7.1.d of the Technical Regulations of the FIA Formula One World
Championship state that a radius of 50mm (+/-2mm) must be applied to each front corner of the
reference plane. (See Art.) The text of this Article was being applied differently by several different
teams, and a clarification was sought. As a result, TD/033-18 was issued on the 25th of July, 2018 –
which, in essence gave the teams until the Grand Prix of Monza to comply with the clarification
indicated in the TD.
Renault provided a picture that they claimed was of Car 8, which they stated shows that the part in
question, at the start of the Grand Prix of Monza was not in compliance with art. 3.7.1.d of the FIA
Formula One Technical Regulations, as clarified with the TD, and therefore they protested against
the compliance of Car 8.
Haas submitted that they understood the TD, and that they have been in contact with the FIA’s head
of Single Seater Technical Matters. They submitted the email correspondence, which were also
provided to the Stewards by the Technical Delegate. They provided details of their new solution and
sought acceptance of that solution. In their final email in the chain, they provided both drawings of
their solution and stated “Given the forthcoming summer break, we will endeavor to introduce this
upgrade for the Singapore GP, but will be somewhat at the mercy of our suppliers so we would
request some flexibility in this matter.”
It sounds like the FIA were aware that the floor was potentially illegal, but Haas were unable to fix it before this weekend's race. It also sounds like they perhaps weren't the only team that had been running an illegal floor, but that other teams had now met the requirements. With Haas not doing so, they left themselves open to protest from other teams, and this is what happened. It should be noted that Renault do benefit quite a bit from this as Haas are their nearest rivals, and this disqualification gives Renault an eight point lead.