We didn't have any dry or super hot spell like we usually get, so perhaps the abundance of water does play a key part.
My snails are the round ones, not the conical ones. They're all over around here, find a snail? It's almost always one of these. I haven't seen the giant crowd in a while, just random stragglers of about 10-15 snails. They climb up this one brick wall, but it is a very moist location, with an old wooden piece of furniture that is disintegrating and turning to mush. I've also seen smaller ones (maybe just babies?) on our back window, which overlooks a very shaded, wooded ravine. It is funny to see all the random trails they leave on the windows, you can see where each one had been.
I don't have a picture, but they are very similar to these dark grey ones with thin, translucent shells.
https://www.google.com/search?q=Oxychilus&tbm=isch Apparently called the glass garlic snail? Idunno, I just know that I crunch a lot of them just walking from my porch to the street.
The slug that hangs out around here is about half that size, with less striking dark spots than the one you posted earlier. He usually hangs out on the cement porch.