I'm only guessing, but I wonder if it could be fertilizer burn, or maybe chemicals in your water. If I use tap water on my plants they get burned, tho' it's been long enough I don't remember what it looked like. Some people thought I was peculiar when they found I bought water for my plants and drank tap water myself. Now I have a rain barrel, which has probably paid for itself in not having to buy water! Could also be something in your potting mix.