Having had flat combs and bumpy combs over the years I have an idea that the irregular are linked to erratic or sudden heavy flows, and the ability of a colony to manage income logistics: to draw foundation quick enough, ahead of incoming nectar. Such discord would be accentuated if insufficient boxes were available to the bees at the time; it's easy for a beekeeper to be caught out.
The spring flow this year was suddden and powerful and I reckon that if I was bee, I'd find it easier to extend existing comb and dump the nectar, than draw new from scratch. Perhaps the spring balance of young wax bees & older foragers was unable to accomodate the flood of nectar?
Strong main flows over a longer period do produce regulation flat combs, and colonies would be likely to be able to deal with the income in a more orderly fashion, and at a time when the colony has peak workforce and more boxes.