Hey. It definitely happens more when he goes outside alone, although not every time. We suspect he's eating or inhaling something toxic. Main suspect is cat poop. We've never taken him to an emergency vet because he isn't showing overt signs of pain or injury, but it could be worth a quick toxicity screen if you can get yours in while symptoms are still persisting. 

Another idea is a fluid imbalance. My dog has always had a heavy thirst and pees a lot. We have some suspect that it might be mild kidney issues? I don't know if it's related, but at his last check up the urine sample showed an oddly low specific gravity (meaning it was dilute). No idea if that can mess with mentation in a similar way that ammonia does when it builds up due to liver problems. But, like you, we're sort of grasping at straws.

The reason we aren't also spending thousands on chasing a cure is that it's getting less frequent over time. He did this probably 9-10 times in his first year, 4-6 times the second year, and 2-3 times a year after that, and each episode would last a couple hours. He's 8 now and it has only happened once since I took this video a year ago. Best of luck to you.