Can you help identify how posterior dislocation happened?

[Clairebeary]

There was an altercation between my ex husband and I and I ended up in hospital with a posterior shoulder dislocation. Although it all happened very fast I know it happened during the altercation. He insists it happened afterwards when I slipped. Please give your unbiased opinion....
The altercation
There was a struggle over my mobile phone as I was trying to call police. We fell backwards onto the arm of the sofa, him over me. My arm was being roughly pushed/pulled back and forth as I struggled to keep hold of the phone and he tried to take it. Suddenly he had the phone and I felt in pain.

The slip
I got up after the altercation to try get my phone back and slipped on some water. I landed with a thud on my bum.

I know this is probably a random question but I really need some help in helping me to decide if I should take this further.

Thanks in advance