1. Ligaments. I think this is the biggest factor. The ligaments will greatly determine the erection angle.
2. Erection quality. If you have a weak erection, the erection angle will be lower.
3. Curvature. If you have a downward curve or straight penis the erection angle might appear a bit less than if it was a upward curve.
But really, I think it’s 80-90% about the ligaments.
If the erection angle is high it means the ligaments that holds the penis to the pubic bone is quite tight and short, creating an upward angle when erect. If they are stretched and become longer the erection angle will decrease as the penis isn’t pulled up towards the pubic bone as much. This would mean that people with higher erection angles might have more potential for lig gains since their ligs is probably tighter and shorter and holds more of their dick inside the body ready to be exposed by lig gains.