Sudden iMac/printer weirdness

Here is the setup: iMac running OS X 10.10.5, main printer Epson WF2860, second printer Brother DCP7065, iPad, all connected via wifi.

All worked smoothly for a long time, made no recent changes to hardware or software and one day last week iMac would not print. Tried both printers, could print to neither. Print queue showed the print job and said "Printer not responding." Could print to both printers from the iPad.

Also, both printers have a scanner function. iMac could scan from both printers.

Tried the usual -- removed printers, re-added printers; reset printers and re-added printers; computer off/on, router off/on, printers off/on. No help.

Did not have time to play with this, so when I needed to print from the iMac, I printed to pdf, emailed it to my iPad and printed from there.

Now comes the weird part. This morning I decided to try printing from the iMac just to see. Got same message in the print queue "Printer not responding." I went to my workaround, but forgot to delete the non-printing job from the queue on the iMac. About 30 minutes later the printer started up and printed the job that I had already printed from the iPad.

What were the iMac and printer doing for 30 minutes? Why was iMac suddenly able to print? Thought problem might have fixed itself, so tried again. Same thing. "Printer not responding", then after long delay 20-30 minutes it printed.

Any ideas?