However, to introduce some action by the unknown silver car driver to suggest it caused the event that caused the portakabin to be released, may assist the driver of the lorry. So rather than be surprised by something that is probably a non starter, the police would like to investigate that avenue of enquiry, establish if the silver car was a good witness.
Perhaps they saw it.
Perhaps they saw the standard of riding of the cyclist immediately beforehand, or the driving of the lorry etc.