Emotions are contagious. A laugh and a smile can spread like wildfire amongst friends who share a connection. Likewise, though perhaps not as pleasant, frustration and sorrow can spread amongst those who are close. Not only that, but he is threatening the mental well-being of your children, which is reason enough to feel stressed.
I do not know either of you, and I've only a bit of information to go on, but if I were to make a guess, it would be as follows:
1) He likely is extremely impatient and "blows up" when dealing with the children because, I assume, they require extreme patience. That would explain why they are the targets of his fury. Not to mention, it seems most people with anger issues are rather impatient.
2) He perhaps feels some sort of regret for his behavior, but attempts to convince himself he isn't doing any harm to anyone but himself. I would guess his line of thinking to be "Am I supposed to not feel angry? Why does it matter to you if I do-I'm the one who's angry!"
I would hope he is aware of the damage his outbursts could do to the children though...
I sincerely think seeing a therapist would help him, as (s)he could help him discover the root of his anger. Having anger issues makes life dreadful for the person affected and everyone around them.
Again, I don't know either of you, so this comment may be useless.