GA agent could be following YBYL (you board you lose) and acting like everyone ahead of you on the upgrade list who had already boarded had effectively declined the upgrade. When it became obvious that some passengers in C+ were going to miss the flight, the GA offloaded them and gave the resulting C+ seat(s) to the highest people on the C+ "upgrade" list who had not yet boarded.

Note that on some DL aircraft 10D could be a seat that's normally assigned at the gate to passengers with special needs (handicapped, service animal, etc.) and the GA could have been keeping it empty until the flight closed to further sales, normally at T-30 or the airport's check in deadline.

OP was lucky to be in the right place at the right time.