Tumor on knee, result of injury?

I injured my knee, the story is under "Knee injury led to cyst". Long story short, what the orthopedic doctor thought was a cyst in not. I was referred to an orthopedic oncologist and he confirmed that the mass that I have is a tumor. I have surgery on the 18th of September, finally the pain will be gone!

What is really weird is I tore my MCL and after that happened I had this pain in a totally different part of the knee. My first doctor dismissed it as a kneecap tracking problem, told me to do exercises, ice and pretty much blew off the fact that that it still hurt (in fact the exercises caused more pain).

What I don't understand is how on earth would this injury cause a tumor? Maybe it didn't, but it sure is a coincidence that I never had this pain until my MCL tear. My new doctor told me that a tumor can be a result of trauma. Has anyone heard of this? I'm not sure how a tumor can start growing because of an injury. ???