I haven't ever seen them block seats on the seating chart (other than the C+ bulkhead seats for handicapped folks and the last N rows of the plane for gate control). I'm not a Delta apologist and anything is possible. Therefore these are just guesses based on things that do happen frequently and offer a less "conspiratorial" explanation:

1. Frequently a message indicating "only N seats remaining at this price" appears on the booking page. That's true at that point in time, but it does not prevent other fare buckets (and in fact lower fare buckets) from being opened later.

2. It's possible there was an aircraft swap? If your original flights were on a smaller plane and your new flights are on a larger plane it could look like there is relatively more availability.