"Special" format in one of my folders

I have several email folders which I use to sort mail for the short term. These present like the inbox, and I am happy. However, I have a folder named “keepers” which has about forty emails that I consider worth keeping for the long term. When I open this folder, it has just recently decided that it would now display the list of emails with “topics” which are in title bars that have a drop-down arrow that then shows the email. The “topic” in each of the title bars is a copy of the subject line of each email. So, what I have is a list of open topics, each with an email under it with a title that matches the subject of the email right under it.
How do I get rid of this and just be a simple list of emails like the inbox??

It seems like the list may be sorted by Subject. Click on the column title and change the sort order.