I don't know why it pops up (only happened to me twice) but you can get rid of it by closing out the app (double click home button and swipe the texting page up to close it out). When you open it back up it should be back to normal.
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I've seen this a couple of times myself. I would do a backup and restore the device to factory and see if your continue to see this happen. You can always restore the device from backup. Also, make sure your on iOS 9.0.2. Good luck.
According to posts on the Apple forums it appears to be a small bug that existed even in the beta version. It's random and isn't something easy to replicate but the consensus seems to be what I suggested (closing the app out) is the only "fix" for now.