This is why I don't use tie outs or exercise pens for my dogs. If they are outside, they are on leashes and I am holding the other end.
The reason?
Because as long as I am holding onto their leashes, I am aware of their behavior and will know IMMEDIATELY is there is a problem. Tying them out or putting them in a pen allows your attention to stray and that's when something bad is going to happen.
I sympathize with you, but once your emotions are in control, remember the events. Your dog broke it's collar and attacked another dog. While the even did not result in major injury, other people have a right to be concerned.
As a side note, don't ever use a choke chain with a tie out - very easy for that collar to get twisted and choke the dog to death. A choke chain should only be used when the dog is in your immediate control and your attention is dedicated.