Container tomatoes, and especially upside-down tomatoes, are very hard to keep evenly moist. The upside-down pots tend to be five gallons or less (at least the ones I've seen), and five gallons is my borderline value for the smallest container that can be watered effectively, especially in hot weather. So that alone may be a lot of it.

If you do this type of pot again next year, look into some of the inert soil polymers they sell to help with water retention, those may help to even out the moisture levels over the course of the day. Ultimately, though, when you grow upside-down, you're bucking nature... some plants will be more forgiving of that than others.

Hope that helps.

--Alison