I don't understand why my wife gets so angry with the kids!

My wife and I have 5 kids, and sometimes I feel like one day I'm going to wake up and find that she snap and hurt herself and these kids. From the 12-year-old down to 3-year-old, get yelled and curse at for some of the simplest things. I know my babies can be a handful, but my God, why does she always have to resort to verbal abuse? She says some of the meanest things to our kids at times. So I'm asking, is this normal behavior for a mother?