It could be myoclonus.

Welcome to Connect, @debodun. I don’t think it is common for people to fall down when suddenly startled. Does this happen frequently to your friend?

I have no way of knowing how often it occurs. This is not a person I am really chummy with, so not likely to share if it did. It’s just that it happened while I was at his home the other day and someone knocked on the door. He was standing up at the time with his back to the door and when he heard the knocking, he just fell down like he fainted (but he didn’t faint). It took him bout 3 minutes to get back up again. I couldn’t help him because of my back problems, but I saw he was conscious and moving. But when he stood up, he was livid, accusative and verbally abusive to the other visitor.

It sounds like a seizure, a bad one.

But why would that make him become so belligerent?

Fear, adrenaline reaction of fight or flight response to losing control; especially when he does not know what happened.
As an amputee, I know that a sudden, uncontrolled fall is quite frightening. Here is an article about it:
ncbi.nlm.nih.gov/pmc/articles/PMC3207805/