Why is my dad so pissed at every thing?

Ok so my dad gets so mad at every little thing my mom does or says and takes I personally. My mom is really nice and our whole family is Christian. We took a vacation the pAst week and I don't know if it was her parents ( cus there old and cranky) or he was just tired or what but my mom didn't mean anything she said in a bad way. (before the vacation he didn't get mad that much) he was abused horribly as a kid. He is usually really nice and funny to me and my sisters and loves spending time with me because I'm the oldest(14) and a guy sO I understand alot of stuff he went through. And he would never hit any of us or my mom maybe in fear of ending up like his parents. Is he mean to my mom because of his abuse or what! I get so confused and pissed off because sometimes he's such a jerk but then he's all nice? Plz hlp!!!