Apparently the iPad has failed to send some email at some point. If you have some emails that fail to send for one reason or another the iPad will try again each time the Mail app is opened.
You can see what these emails are (if you are fast enough, or then fail again). Tap the Mailbox button at the top of the email list until you see a list of Inboxes and Accounts. If there is pending/outgoing email there will also be an Outbox. This only shows up when there is outgoing email on the iPad that has failed, or is still in the process of being sent.
Open the Outbox (if visible) and look at what's in it. That may explain things, or not.