I see this in Texas during the middle of summer - and always put it down to heat, as it happens even with lots of watering. Some varieties handle the heat better than others (eg. 'Gypsy' bell does well for us).
I find fruiting is early summer before it gets too hot, and then from September through to the first frost. So during the summer heat, I just keep them watered and make sure they grow, so I have nice large, healthy plants ready for when it cools sufficiently for fruit to form.