Sheer luck. Nothing to do with what you did. Just how far behind a 50mph truck were you to enable you to speed up to "fast moving". The rock could have bounced lower, bounced even higher, bounced to the kerb.
The simple fact is cars have improved massively, driving skills have not. Something has to be done. You only have to look at the amount of accidents on the M6 roadworks in a 50mph limit while being changed to a "smart" motorway. Speed limits are just that, a limit and not a target! Because it says 40mph does not mean everybody has to do 40mph. The slow cars pay road tax and the ferraris pay road tax.
It is a sheer lack of courtesy and patience that cause most accidents, as can be seen on many a dash cam warrior website.