I was forcibly deep throated and can’t understand why I was unable to stop him.

I had an experience where I was tied up (consensually)and deep throated. I agreed to oral sex but I wasn’t expecting him to go that deep. I couldn’t breath and so I tried to fight him off, but he ignored me. My question is, why couldn’t I close my mouth or bite him? Is there a physiological reason, because as I remember I tried everything to get him to stop.