It is the unknown, the unpredictable.

A copy/paste of your link:
the reward or punishment has been applied irregularly, then a second condition has been created where, upon receiving the stimulus, the person forecasts and imagines the reward or punishment being applied.

Obviously, ex-h knows he has go be calm in order to have contact with me but his temper or reactions are not always good. When his voice escalate, i shut him out. ( my boundary ). His reaction get worst, he goes mad, hits, scream, try to break doors etc..

Then, he apologizes to the kids by saying he does not control his anger around me. ( making me the reason of his abuse ).

To me, no apologies. A simple: i snapped..

Well what if he loses it to the point of shooting me? Then what? I made him do it? He had no control?

I fear him... i do not trust him and i am hyper-vigilent when it comes to him. Just hearing his name from the kids shuts me down.
The more they talk about him, the more i boil inside.

My anxiety is in my chest. Huge knots with a string getting thight towards my heart.

To calm down, i take an anxiety pill or i step out to cry. If i can' t calm myself, i go to emerg.