I’m no dog professional but it could be possible that meeting other dogs on leash can be an anxiety producing situation because they’re restricted and feel trapped. Whenever dogs may try to interact with him, he could feel threatened by them being in their space and a leash unforced smaller boundaries and less of an escape route if the dog feels confronted or such. I would avoid any random leash encounters with stranger dogs out of safety for a long time. Until then it could be good to introduce familiar dogs through fences and stick to the same particular dogs. It’s important to have positive reinforcement and distraction in stressful situation. Give the dog a moment to sniff then detach him the dog from the others by treats and pets and positive affection. Keeping the dogs busy in their own world and yet in a coexistive environment is essential. Make sure they have their own space.My dog is a 2 year old rescue. He’s very reactive but we have been working hard on his training. He loves the dog park, and goes on adventure walks with a group of dogs each week. But on leash is a different story. It was never a huge issue, but recently has becoming worse. Things will be going good, tails wagging, they greet and have a sniff and then 10 seconds later my dog switches and attacks and we’re all caught off guard. And then sometimes he’s completely fine! It’s unpredictable and there are no negative signs leading up to when he’s going to switch. The leash is loose and sometimes I talk him through it which I find helps. Any advice on how to approach this? Thank you!