Adverse reaction on return from injury

Hi all,

I work in elite youth football (physiotherapist) and recently a player who I had been rehabbing for 5 months following a lateral meniscus tear (sutured but not removed) and a torn ACL (no surgical intervention), returned to training. For several weeks prior to return he had trained non-contact, with some sessions of one-one contact practice (attack/defend drills) and lots of end stage C of D and plyometrics.
Prior to his 1st full contact session with the team the Academy lead physio, club doctor and players own consultant all cleared him to return.
Unfortunately in that 1st session back he went hard into a tackle and landed with a twisting MOI on his affected side. His knee immediately swelled up and there was particular tenderness at med JL. ACL laxity was positive but he is also hypermobile.
2 weeks later and he no longer has any signif signs and MRI was negative.
My question is how can he have suffered such a bad reaction without any actual reinjury and does anyone else have similar experience when returning a sportsman from L-T injury.
Hope you can help, I am looking to investigate this topic further
Regards