Why Did The Gastrocnemius Tear Rather Than The ACL?

I'm an orthopaedic physical therapist but asking about my own condition. Recently it seems probable that I have Marfan's Syndrome. I've also had numerous other injuries, including a C2 fracture (mild spinal injury, but the odontoid ligament is insufficient) a patellar ligament maceraction S/P reconstruction from a low velocity head-on collision on one side, and a spontaneous tib/fib fracture dislocation of the other side (S/P ORIF). My question is in regard to an injury of my posterior knee that happened 10/31 which was an EXACT copy of one that occurred in 2009 on the other side.

This is what occurred. I stood up and leaned forward slightly to get onto my bed when suddenly my knee snapped backward into hyperextension and I felt ripping at the medial gastrocnemius head. I fell forward onto the bed as the ripping occurred. I went to the ER for the first time this occurred, and the doctor had no idea what had happened. A resolving bruise appeared a few days later. I eventually healed, being careful to rest and ice the knee gradually increasing ankle and knee mobility to WNL. It has happened again now on the other side. Here is my question: Why did my gastrocnemius tear rather than the ACL, and why didn't the ACL protect me?