This may stem from rumors of Season 19 winner Josh Martinez being a part of the season, though he was seen in a video partying in Miami in a crowded night club a couple of weeks ago.
The word is that the casting interviews have been postponed because the contestant list has not been solidified as of yet.
It logically tracks that a COVID diagnosis would force one or more casting changes, though it should be made clear that there are no confirmations at this point that anyone is sick.
No one seems to definitively know why, though fans have speculated that one or more of the potential housemates that are said to be in quarantine has tested positive for COVID-19.