Maybe they are testing the new "Purple" board, previously unknown and not announced?
I am of course being a wise-acher.
I have seen that occassionally, but not to the degree you captured. Yes, it was a very hot night for electricity last night.
I spent much of the evening around civil twilight until midnight feeding the mosquitoes and watching a slow moving series of thunderstorms that weren't embedded move through and try to get some pictures.
I'd be interested in knowing what settings those stations had. And if they were H field rather than E field detections. Maybe the up-to-date hams that know a lot about low frequency propagation would know if the 'bands were open' or not. Not something that I've studied as a ham, but know there are 'conditions' better than others and maybe there was a lot of ducting to bring those in. I recall when TV was FM, non-digital, that during nights like last night that we usually watched Channel 4 WCCO from Mpls/St.Paul with the beam pointed to the west to hear them from 90 miles away,and the signal would gradually degrade and then we'd begin to see a less powered channel 4 from somewhere out in the Dakotas if I recall come in for a brief time, then shift down and the 'CCO station would start to be visible in the noise. Of course now all we see is pixelization when channels up in the UHF begin to drop below needed levels to decode. But it still happens with weather that is hot and humid, and moreso when a storm is between us and the transmitter. But not closer stations.