I don't know what the rules are. The collections or authors I see tend to fit one or more of:
- Collection or author I opened the current book from.
- The author for the book I am currently reading if it was opened from other than a collection.
- The author for the book I last finished. This mainly happens between books, but, I think it can last a little while into the next.
- Collection or author with at least one unread book.
- Some random collection.
In other words, no real idea. And I have been thinking about it lately. For some reason my "Kobo" collection (built from a calibre column for where the book came from) has been displayed for a few days. The last purchased book I read, and is in this collection, was finished on the 7th. And I'm sure I haven't opened it since then.
As to pinning a collection or author there, I don't believe there is a way. A long time ago you could do it in the database, but, that method does not work now.