More nitrogen, i'd guess. paling from bottom-up. one of the more unique symptoms.
if it's one or two, maybe ignore, but the plant in pic 6 has more than a couple showing the symptom. Plants do shed a leaf occasionally and it'll suck it down before it falls off, but anything with more than 1 or 2, i'd add a little more N (one way or another, lots of options) around the base of those plants.