Yep, I hate mysteries too.
What I can't understand is the hard disk activity when there are so few files scanned.
Something I would like you to try is to set Updates to Manual (avastUI, Settings, Updates), it is possible that if you have a broadband connection available early in the boot, avast could be checking for updates. Generally this would make itself noticed in the boot being a little slower, but I don't know if hard disk activity would be great.
You will get a nag in the form of an ! exclamation mark over the avast tray icon this can be temporarily disabled, avastUI, Settings, Status Bar, unchecking the bottom three options relating to updates, see image, click to expand.
These can be reversed if it doesn't resolve it; if it does resolve it there is something that can be done to delay the update check, allowing you to set those settings back as they were.