The correct answer to this open question is the following.
What is in the preface to Copernicus’ book De Revolutionibus that leads scholars to suspect that he did not write it is the fact that those written ideas seemed fictional and not according to the nature of Copernicus investigation. That is why scholars suspect that probably it was not Copernicus the one who included those ideas in the first place.
The motivation that Copernicus (or whoever was responsible for the preface)would have to include such a statement would have been that knowing that the Catholic church showed no mercy with impious ideas that contradicted the teachings of the Bible, probably that was the motive to try to deviate the Church's attention and protect Copernicus of being persecuted or sent to the fire.