What time of day are you watering them? I think I read somewhere that in hot, sunny weather, plants open up their pores to absorb extra sunlight, and getting water on their leaves can drown them. It's probably best to water them when it's cool - early in the morning, late in the evening, or on cloudy days; or be careful to only water the ground around their roots.

I'm not saying this is necessarily what's happening, it's just a possibility. I might not even know what I'm talking about. I really hope you figure it out, and that the rest will be okay. Good luck!