I suspect at the time the Freedom of Information Act was brought in in the UK, there was a large amount of panic in lots of government and government-related organisations which lead to lots of stuff being inappropriately classified, which may include this report.
As an aside, the fact that older AAIB reports aren't readily available unless you know exactly what you're looking for, or (as in the case of the Tornado/C150 mid-air) incomplete is more of a problem, in my view.