I would agree with going and talking to the owner of the nuisance dog. If you do it politely and they are (hopefully) responsible dog owners, then they are going to want to know. The fence could be faulty, the dog could be sadistic, or the battery in the collar transmitter could be dead.. lots of variables and a good idea to get to the bottom of it.
I also agree with your response to the situation. I watched a pair of Labs rip a cat in half literally by playing tug of war with it. They were oblivious to my yells and by the time I found something effective to get them away with, it was far too late for the poor puss. Stereotyping a breed will only lead to heartache- much better safe than sorry. Thankfully there were no injuries and hopefully you wont have to deal with something like this again!