Don't beat yourself up too much, we all lose control from time to time. In my opinion, because your son is so well behaved at school and only does this at home, I don't think there is an underlying issue. I think it's a behaviour that there has been no consequence for. I think you should breifly explain in child terms to him why you had the outburst that day and you are sorry. This behaviour will not continue though and from now on "insert consequence" will happen if you are yelling. Stick to your guns and always follow through from the second it starts. At first it will be tough to do but it should put a stop to the behaviour. Best of luck. Hang in there!