Maybe they liked the dirtier water?
I don't have much experience with pond snails, I've only just acquired some and since I did not want them in any of my tanks I pulled them out and stuck them in my 1 gal tank. What I do have in my other tanks are ramshorns, malaysian trumpet snails, and 2 nerites in the 5 gal. I've never seen my ramshorns do as you describe in any of my tanks, nor have the nerites. It's the MTS that have done so on more then one occasion. I never did figure out why they do it, it's always after a water change, the betta and nerites that are in the tank with them are not acting strange and do not seem to be upset about the water change, it's just the MTS. The first time they did it I about panicked about it like you did, thinking I had nuked my tank someway somehow, and my betta was going to be the next to start feeling the effects. Now I just ignore them, if they want to crawl up the glass and hang at the top of the tank because I put clean water in the tank that's fine. They generally head back down to the substrate within an hour, and none have ever died from the water change.