dislocated knee and fractured femur

A couple of weeks ago I was getting my son out of the car when all of a sudden my knee cap popped over to the side of my leg and I fell over into my car. The ambulance took me to the hospital where they gave me alot of medication and popped my knee cap back into place. The doctors said that I dislocated my whole knee, and gave me crutches and a knee brace, and also told me to put no weight on the hurt leg. I had to have an MRI which showed that I fractured my femur, now I have to see a specialist. But what I am wondering is how did all of this happen just from getting my son out of the car? I have been laying on the couch for two weeks (since I can't do much else) and worrying about this. The doctors do not seem to know how it happened unless I twisted my leg just right, but this whole thing seems quite odd to me, because I thought there had to be a lot of trauma to dislocate a knee and fracture a femur.

[Dr Vinod]

Chances are you could have a stress fracture previously that would give away due to this twist. Sometimes the twist is very strong in the opposite direction to cause this but this is a rarity.
The Orthopaedician is the best person to diagnose the cause after doing the required Scans/MRI .
More that that immobilisation of your joint and cure of your present problem is important. Please take calcium, protein and balanced diet  and take all the medications prescribed to you by the clinician
Take care!