Just thought i’d add a little.
Poor ankle mobility as mentioned in the article posted by Ash is I believe a massive contributer. Has the athlete a history of ankle sprains and therefore compensating at the knee?
Articles written by Mick Boyle and Gray Cook are very useful. Shirley Sahrman also mentions poor ankle mobility as an issue.
Exercises are simple and could be done in a minute in the warm-up.
Just another thought.
Jim