Why does the timeline expand for no reason?

ok .. thanks. This is not something that happens when I first create a new slide. I am working in the slide ... the timeline is whatever the default length is ... I add a couple of things ... it's still fine ... and then somewhere along the line, the length of the timeline just out of the blue changes. I'm realizing now that I usually notice it when I go to preview the slide. I notice then that my seekbar is much longer than it should be ... and when I leave the preview mode, I see that the length of the timeline has changed. Also, if I run the slide from within development mode so I can add cue points to the timeline, it will take a long time to start to run ... probably because the timeline has made itself several minutes long.

So it seems to be preview and running the slide that are triggering this to happen.