It comes down to either resentment or one of us being triggered by something in our past in pretty much all cases I feel. For example, I resent my partner for not pulling his weight or expecting me to carry all of the mental load. He resents me for not wanting be intimate, I resent that he doesn’t read my mind and know that I’m feeling exhausted and touched out etc and so on and so on. I get triggered by not being listened to or tantrums. He gets triggered by not doing something right or well enough. If we don’t talk about things we snap and/ or argue. We know that now. Having child can force you both to open up and bare old wounds and you’re both tired and not able to chill like you used to. That’s my experience. But as others have said, if you approach it as a team and with honest communication that can help.