I came across this very soon after it started and have spent hours trying to fix it.
But have only just come across this thread.
To me it is a side issue since I am really trying to fix the Windows 8.1 disk activity 100% problem that so many have also reported. I did not notice the issue when updates were set fully automatic and was obviously hoping the continual updating accounted for the 100% disk activity.
I am just glad others are now reporting this particular update issue and it is not just me!
I have tried manually updating using AntiMalware Definition Update, Fixit on Updates, Windows 8.1 troubleshooting updates (it always reports errors and then says they are fixed), clearing SoftwareDistribution and caroot2, I have run Disk Cleanup but only after noticing the problem (as part of attempt to fix), have also run updates from Defender itself.