Thanks for the response and offer.
I let CCleaner delete them and will see if I get anymore. I don't use that computer all that often so it may take a couple of days to see if they are re-created.
I got no indication that the .dmp files were there other than when they showed up in CCleaner so there was no BSOD or any record of them in Reliability History etc. (I didn't look at the Event Logs).
Just a point of interest, when I googled "USBXHCI dmp files", There were only 4 hits and they all seemed to be memory related.