I don't recall where I read this but I have read an article that engine surges can send supersonic shock waves forwards through the fans and then in an instant the flow reverses again. The effect is like bending metal very quickly in two directions, just like the forces which snapped the tail of that AA A300 over New York in 2001. The result (so I read in the article) is to cause minute hairline cracks in the fan blades which will propagate.
Worth asking some engineer types to comment?