M.P.
How old is your son? If he's school age or especially if he's preteen or a teen, I agree with Denise S. The more you push him to talk to less likely he is to talk.
You also don't say what he did when he was angry. Sometimes anger just needs to be let go of and not ever discussed. Is he still angry? Is he acting out? If the answer is no to both those questions, I suggest you let it drop.
If you're talking about your 8 yo, it's normal for him to be angry. You asked a question about how to get him to do chores. I didn't read your question or the answers but I wonder if his anger isn't the normal anger of a child who is in a power struggle with their parent(s). You don't really need to know why he was angry, even if he did something serious, like hitting you. You need to let him know it's OK to be angry but not OK to do whatever he did, if that's the problem.
I suggest that when you focus on the reasons rather than the behavior you get side tracked from the real issue which is how to manage feelings. He may not even know why he was angry. Insisting he tell you why adds another layer to the power struggle. I'm suggesting a power struggle because this happens often with children. We want to control our child and they want independence. Learning how to discipline so that both parent and child feels empowered is difficult.
I suggest you read How To Talk So Children Will Listen & Listen So Kids Will Talk by Edna Faber and Elaine Mazlisch. She writes so that we can see both sides of a conversation and gives examples to help learn how to use words differently, thus empowering the parent and child.