It's almost certainly Contacts / iCloud doing its accounts check or Safari doing its preload for the default bookmarks/most recent pages.

Yeah I also thought it should be something like Safari preloading pages.

Still, why must the page preload happen during an update?

There's plenty of time for that after the update..

Because all the default services are preloaded during setup. iCloud and Safari both have an update script in their package installers that 'prewarms' them for your first setup.

Ok, that makes sense. Thanks!

I had about a dozen of these appear during upgrade as well - so it's not just you.