Why do Sofia and Harpo argue?

There is a major conflict between Sofia and Harpo, two characters in Alice Walker's novel The Color Purple. What is the cause of this conflict?
First, let's establish the relationship between these two. Relatively early in the novel Harpo meets and falls in love with Sofia. After she becomes pregnant, they get married; they have several other children in quick succession. But they won't stay a couple for very long: Harpo wants to dominate Sofia, but Sofia refuses to be dominated. Their arguments get physical, and she ultimately moves out with the children. Here, another character describes their fighting:

fighting like two mens. Every piece of furniture they got is turned over. Every plate look like it broke. The looking glass hang crooked, the curtains torn. The bed look like the stuffing pulled out. They don’t notice. They fight. He try to slap her. What he do that for? She reach down and grab a piece of stove wood and whack him cross the eyes. He punch her in the stomach, she double over groaning but come up with both hands lock right under his privates. He roll on the floor. He grab her dress tail and pull. She stand there in her slip. She never blink a eye. He jump up to put a hammer lock under her chin, she throw him over her back. He fall bam up gainst the stove. 

What causes this kind of fighting? Sofia isn't like a lot of other women of the day: she is assertive by nature, refuses to work as a maid, and wants equality with her husband. She is not afraid of men, and she's not shy about defending herself either; she's willing to fight to protect her own rights.
Harpo, meanwhile, is an otherwise kind man, but he is frustrated because he can't control Sofia or make her "mind." Maybe, Walker suggests, Harpo doesn't want to control his wife as much as he wants to live up to socially imposed standards for how men and women should behave:

Harpo want to know what to do to make Sofia mind. He sit out on the porch with Mr.——. He say, I tell her one thing, she do another. Never do what I say. Always backtalk.
To tell the truth, he sound a little proud of this to me.

Long story short, it is both the natural personalities of these two characters and the social expectations for each of them that causes the conflict between Sofia and Harpo.