Delta has done similar stuff many times when they have overbooked flights.
They try to get the people with minor issues, delay them until last minute, and then force them to bump to the next day.
I had personally such an issue back a few years ago, where they held us up till 55 minutes before the flight, and then suddenly "they couldn't print boarding passes anymore, because it's less than an hour before the flight". We tried begging and arguing, but nothing doing. When we went to the supervisor for help, she refused with much nastiness. Another passenger on the plane told us later that she overheard the supervisor telling her: "don't you dare let them board, we are 17 people overbooked, we need to kick off whoever possible"
Another time a friend of mine arrived 55 minutes prior to a domestic flight, and denied boarding "because he arrived too late", but surprisingly allowed him to board a connection flight which departed 10 minuted earlier.
Holy crap!