Accusation of motor vehicle liability damage despite innocence

Hello, I was accused in July this year of having caused a rockfall in a two-lane construction site due to an overtaking manoeuvre with excessive speed. However, since I still have a good memory that there was a lot of split on the road and I did not want to damage the vehicle myself, I kept exactly the speed limit. So I did not overtake anyone and certainly not too fast. Quite the contrary: I was from some other A utos overtaken, which in turn were considerably too fast (for the specified speed limit). I also didn’t notice anything about the incident on the ground, since no one has made me aware of anything. Only 2 weeks later I received a letter from my car insurance company, where I had to comment on the incident. I described the whole as here and that I was not aware of any guilt. In the meantime, I was upgraded in my liability yesterday I received a letter that the plaintiff had a witness and that my insurance now regulated the damage 100%. But how can it now be that a definatively lying (!!!) testimony leads to me being an innocent person sitting at my expense of the upgrade? Then could anyone do that, who has his stone stroke replaced free of charge? n. Just write down a license plate in a construction site and get in touch with his insurance company. And if they cross the line, just pull the brother in as a witness and let him lie as well!? Can you help me how to proceed? Thank you! Greeting Alex