Open the file. Look at "Sequencer" check: unchecked.
Ctrl-Right, go to VSE screen. It has another scene there. That scene, has "Sequencer" checked.

Try:
Come back to "Editor" and look at Sequencer: it changed to ON.

If not:
In VSE screen, render the image. Then, go to Editor screen.

How did I find it? I need a scene that has some strips in VSE (for example, a sound to synch with animation). So, I created another scene to make the final Video Sequence. In first scene, I don't want to render through Sequencer (maybe it has more stuff that I don't want to render). In the VSE scene, I imported the first scene and, doing that, I found that Sequencer check changes automatically.