Environmental stress.

Did you recently put them outdoors? If so, you need to gradually harden them off, exposing to a little more sun each day.

Thanks, Jean.
They've been out for just over a week now. The sun hasn't been overly bright or anything. Infact most days have been pretty shady. Will they be okay?

Difficult to say.

Did you harden them off before they went outdoors? If not, even cloudy days can be tough on seedlings started indoors.

Recall that previously they were coddled indoors, now they are out in the big wide world -- low humidity; breezes; and extra drying.