Ruff ruff bites another dog

Yesterday, I took the dogs for another walk down behind our houses. I normally walk them down to behind the houses on a lead, then when I check there is no one around I let them off for a run. I think it is the best thing to see them running free. Normally we never see anyone down there, but yesterday Ruff ruff ran ahead of me around the corner, then suddenly I heard a man yelling and our dogs running back towards me. The man yelled at me that Ruff ruff had bitten his dog ( a fluffy white dog) The evidence was clear Ruff ruff had fur in her mouth. I was very sorry for the guy and annoyed at Ruff ruff. It is awful to have your own dog attacked by another. Thankfully there was no damage and the guy calmed down. I am not sure why Ruff Ruff attacks other dogs, I think it is out of fear. I suppose I must be much more careful where I let them off the lead. Officially the only place you can do this is at a dog park, but Ruff ruff does not do well at all at Dog parks. I am unwilling to always have them on a lead as watching the dogs run, and knowing that they will always follow you is one of the great joys of owning a dog.