Massive - The reasons for arguing are completely unrelated to any event that has happened today. That's the problem. It's just part of a pattern (which is how you knew it was coming) based on his own feelings and emotional immaturity about things and situations which are completely out of your hands.
Example:
With your 'D'P, is sounds as though: Things to do with spending time with his kids = argument.
With my ex: Things to do with spending time with his Mum = argument.
He found being with his mum stressful for many reasons, and he took this out on me. It didn't matter what I did, how thoughtful I was, how kindly I behaved towards her, on the way back in the car, he would start picking me apart and ranting on and on and on and fucking on about my faults. How I was selfish, how I made it clear I wanted to leave, how I clearly didn't want him to spend time with his mum - all of which was utter utter rubbish - just because HE felt shit.
It was NOTHING to do with me, but i was there, so I became the person to blame.
Ditto when something, anything happened in his life, it was a massive fucking drama. When a my mum became very ill, it was a non-event because "she's old, these things happen. Get over it" 
Please please don't marry him.