You'll likely quickly build up an increasing list of messages that you sending out via Engage. That can include a load of historic or even test messages.
Messages are grouped by send at date, or send from date for messages set to be sent between two dates. When grouped, they are sorted by date, descending. So the oldest dates are at the bottom, newest at the top.
Here's a useful tip on how to limit the messages displayed to only those that are "live" (i.e. not message campaigns that have already completed), to unclutter the list.
Simply uncheck the "Complete" box on the message view dropdown, like this:
Clearly you can also use these checkboxes in a number of other ways - for example to just show draft messages (i.e. requiring approval).