The Crop Tool in GIMP has a number of options available and one of these is the Allow growing option. Many options of the Crop Tool are quite intuitive, but the Allow growing option does require a little explanation in order to understand how it works.
If you draw a selection using the Crop Tool without Allow growing selected, you will see that the edge of the selection will not extend beyond the edge of the image. When Allow growing is selected, you can draw a selection that extends beyond the edge of the visible image. The resulting image may vary depending on whether part of an existing layer extends beyond the visible image area.

