Yes it can be difficult to work out exactly what happened. I explain in lectures that my investigations are like looking down a long corridor, with lots of doors. I open each one in turn and look into the room – each room represents a different causation factor, such as mechanical defect, driver illness, drink/drugs, road surface, other road user action, weather, etc.. As I look in and decide that that factor didn’t feature, then I close the door and move on, eliminating each cause, until I usually have a couple of reasons left. So it is a process of elimination, down to those reasons that I cannot eliminate.
It is a challenging job, but is very rewarding and interesting. Each crash is different and there is never a dull moment.
If you like science, and forensic science, go for it. Good luck.