Landing in a valgus position can lead to condylar lift-off, stretching the MCL and causing instability.

“what happens when you land in a valgus position is like this right so you land with the leg a little bit like this what this does we call this condylar liftoff right so that's why you often see MCL tears because this gets stretched the conval lifts off of the surface and that surface on the inside of the knee this is called the medial part of the knee is very congruent it's the most congruent part of the knee the lateral side the surface of the tibia is convex it's very unstable in general so now you're only bearing weight basically on the unstable non-congruent part of the knee”