Try holding down the CTRL key and resize the event. Does that give you the same thing? If yes, it's telling you the playback rate has been changed. It's an easy way to speed up or slow down a clip.
Have you dragged that event out longer? The little white triangle shows you where an event loops from the end back to the beginning. By default text events start out as 10 seconds long. Drag it out to 60 seconds and you'll see one of those notches every 10 seconds. The same thing happens with regular video events too, but you're not as likely to drag those out to have them repeat so many times.