Emperors do double damage with siege equipment. Maybe you got hit with a royal rock.
Emperors haven't done double damage to players since... Thieves guild update I believe. They do still deal double damage to walls.
The extra damage is likely from critical hits. Or, perhaps you're just looking at your death recap? The death recap will show all the accumulated DOT damage from one person's ability, or siege attack.
The damage you saw on your recap was the cumulative damage dealt by the dot so its possible for vamps to literally see +30k hits on their recaps even when the siege monkey only sees 10k ticks.
First PS4 NA Grand Overlord, Stormproof, and Flawless Conqueror.
Potato Lord of Atrocity