Signing in. Most websites/apps will probably also grab your unread count, and maybe even your subscription feeds.
Another example is checking your inbox. Lemmy actually has 3 inboxes: mentions, replies and PMs. A lot of websites/apps bundle these three so they will need to check all 3 inboxes via 3 API calls.
From my experience it's entirely random. You can make 5 actions and all 5 will work. Then have a string of 5 actions where none would work.