I have the same thing happen with lavender here. Haven't even tried any Hyssop because the lavender hardly ever makes it through the rainy season unless I keep it under the overhang of the house and then it gets no sun.
They love dry conditions and a gritty, fast drying soil would be my best recommendation if you want to try again. Mix a whole bunch of Perlite in with your potting soil and try again with that mix.
I'm sure it was the heavy rain that killed the plants, coupled with too moisture-retentive soil. Btw beware of "Moisture Control" potting soil. It's fine if you live in the desert, but anywhere that has humidity, it just stays wet all the time and kills plants right and left! The little moisture gel granules suck water out of the air and keep the soil so wet it's amazing. My gingers and cannas love it, but they can live in a pond and be happy. You can also use it indoors, (hey, it will probably help to de-humidify your house!) and anywhere you have low humidity.
