My dogs chased another out of the park

I was able to leave work earlier tonight so when I got home I said to Adam that it would be nice if we took the dogs to the field for a quick run around which we don't normally get chance to do.
When we walked in the field there were a couple of ladies and 2 dogs so we walked away from them before letting ours off lead. I had a ball each for Ben & Max but the 2 dogs wouldn't leave them alone.
We managed to get a ball to each of them which usually means they will ignore anything around them other than us and the ball.
One of the dogs, whose owner I know in passing, picked up Ben's ball and then ran, very fast but it had dropped the ball. Ben, being ball-less chased him and then Max joined in. They were heading straight up the centre of the field, we were calling and calling, then all of a sudden the dog turned and headed out of the field! We were terrified that they were heading straight for the main road so I shouted to the ladies who were at the other side of the field that their dog had just run out of the field!
Max came back first, followed by Ben and the the other lady's dog. She came over to us after and asked what happened so we explained and apologised, but she said her dog is quite timid around new dogs so not to worry, he was actually heading for home. He is a beautiful collie x rescue and she hasn't had him very long. I asked Adam to walk our 2 away whilst I stroked the other dog who was so friendly, just scared of other dogs, at least those who chase.
She offered to take him to the other side of the field but we had ours on lead and told her not to worry, we would take them home because we're not normally there at that time of day so she should be able to carry on as normal.
We walked away and next thing I knew, the same thing is happening again, this time a german shepard chasing him. He went out of the field again and the lady ran out after him but didn't come back so I'm hoping she just took him home and that nothing bad happened.....

Hia
Dont feel bad, if the dog is scared and obviously likely to run away she should of had it on a long line
She should have taken the first time as a warning and it was up to ger to stop it happening again
It shouldn't matter that its not your "normal" time! you have the right to walk your dogs when ever you want!

Bless I hope she found him. Hopefully he went home, looks like she will have to help him get over his timidness.
Let us know if she got him if you find out xxx hope so

Thanks, I will see if I can catch up with her in the week and find out what happened.
I know it's not my/the dogs fault as such, but it was really scary that even with the balls which are the ultimate distraction and focus for them, they just seemed to get this tunnel vision for chasing the dog and didn't react at all to us calling them.
I hope we don#t go through that again!