The determination of how the accident happened is determined by your evidence and more importantly, by independent witnesses who can give evidence as to what they witnessed.
Often, the best evidence is videotape evidence of the incident. Sometimes, there is a need for a more thorough analysis of the physical evidence where there are no witnesses and there may be a need for an expert to be retained such as an accident re-constructionist. Each case is different.
The best thing to do is to collect as much evidence as possible as close to the time of the accident as possible such as photographs, witness statements, reports etc...
If you can get the names and contact particulars of witnesses and take photographs of the scene of the accident, do it. This will help significantly.