Just wanted to send out a quick update. After much investigation, here's what we figured out:

A walk up customer purchased a FIRST class of service ticket T-30 before the flight departed, but no seat was assigned to him / her. The gate agents processed the upgrade list at 52 minutes past the hour, and I was upgraded into FIRST but then downgraded at 53 minutes past the hour so the FIRST class of service ticket holder could have his seat.

Customer service was awesome at investigating this for me and helped me understand what happened. The customer service agent event said "Had I been the gate agent, that guy would have been sitting in coach if I had cleared your upgrade". Delta offered a very nice goodwill gesture, which was not necessary since all I wanted was a good explanation as to why this all happened.