I should imagine it was the drogue. I wasn't aware that there was a "rocket" to deploy anything; the only rocket I can think of on my current seat is the one under me. The main chute's released by the drogue shackle withdrawing, allowing the drogue to head up and drag the main out of the seat box. If the Harrier seat's different, I don't know because I don't fly it
Didnt an UAS stude land in the burning wreckage of a harrier a few years ago?
The story at the time; I was on a UAS around then; was that she was too light and was outside seat parameters. Someone more qualified than me would be needed to say how that could affect her ejection.