iMac restarted 4 times in 24 hours because of a problem
I found this same suggestion elsewhere along with the advice that it might be associated with a USB-connected device.
I disconnected my Anker powered USB hub from my iMac. I had a Sabrent external HDD enclosure plugged into the Anker and it is now plugged directly into the iMac.
I have had odd things happen with the Anker USB hub in the past and was a bit surprised because Anker is not your usual cheap Chinese crap - they get consistently good reviews on multiple Mac sites.
It's been two days now and no more kernel panics; I am just about 99% sure this is going to do it.
BTW, is it "D" or Option + D to get Apple Diagnostics?