ummmm... or you just have bad knees or instability for other reasons. I tore my ACL while playing laser tag with my boys. Nothing extraordinary, just a pivot and whamo... a crumpled pile of pain. Oh yeah, I was shot repeatedly while down. At least it was not my sons.
Long recovery, but you do not need your ACL. I decided I liked to be active and run so I had mine replaced.
Good Luck,
Jim