Who even knows? For whatever reason the board decided to keep quiet, didn't elaborate on its reasoning, let Altman and his allies control the narrative, and rolled over when the employees inevitably revolted. All we have is speculation and unnamed "sources close to the matter," which you may or may not find credible.

Even if the actual reasoning was absolutely justified--and knowing how much of a techbro Altman is (especially with his insanely creepy project to combine cryptocurrency with retina scans), I absolutely believe the speculation that the board felt Altman wasn't trustworthy--they didn't bother to actually tell anyone that reasoning, and clearly felt they could just weather the firestorm up until they realized it was too late and they'd already shot themselves in the foot.