FWIW, assuming this post is not in reference to something that happened a while ago (which is possible) it was probably DL 1894 SFO-CVG and DL 5909 CVG-BWI. This would be scheduled at a 40 minute connection ( likely one of the many ways in which OP's narrative is exaggerated).

On Monday (the flight doesn't operate on Tuesday) the flight was 23 minutes late into B5. DL 5909 left 5 minutes early from B21.

Sounds like a case of life happening and 40 minutes just not proving enough padding for a transcon trying to make the last connection of the night.

Also surprising that Delta would auto-rebook a coterminal.