Pearson was looking the other way when the player clattered in to him.
So Pearson's first thought was that the player had perhaps done it on purpose.
So Pearson got very angry, and grabbed the players throat, but then had second thoughts when he realised that everything he did was being captured on the TV cameras (and remembering Pardew at Newcastle who head butted that player).
So I think Person then tried to "calm down"
However he then held on to the players arm/sleeve and refused to let it go, so I am not sure what that was about.
He may still have been angry, feeling the player had clattered in to him on purpose, but was trying to supress his anger.
I suppose if it was like we were in a supermarket and someone clattered into our back with a trolley. Our first thought would be anger as we span round and we may give the other person a mouthful.
Then we may notice it was an old person who had fallen over, or a person was having a fit, so our attitude would have to change very quickly.
I think that is what happened to Pearson. Still strange though.