what to do?

You have to think about a couple things...

1st. Dogs are animals and although they are predictable, nothing is gauarnteed. For one dog to run into another dog's pen; that could be seen as teritorial and I would not balme or cull a dog for attacking another dog that run into its pen. What would you do if I ran into your house?

2nd. Dont make this a bulldog issue alone. Kinda of goes back to point #1.....I have a plott dog that is 14 months, great young dog. But if another dog runs up to him, he is going to either fight or make the other dog back up, if there is any kind of challenge. I've seen many of old rough cur dogs that would fight to the death if another dog ran up to them. I dont think your issue is becuase the dog is a pit.

3rd. You have a safegaurd animals from each other. I dont know much about the little shaggy dog, but she probably had no buiness going to another dog pen. It is a GOOD idea to make sure that no dog or anything else can get to your bulldog. If the little dogs and puppies are not old enough to have a fair chance at getting away, they dont need to even be close to a dog that might snap (and all dogs can snap).

So, my opinion is leanr from this, and make corrections to make sure it doesnt happen again.