Some sites split the videos into several smaller pieces (e.g. 6 minutes long) and Downie downloads them this way - as several separate parts. This behavior may be unwanted by some users. For this case, Downie includes an option in Preferences > Postprocessing to stitch multipart downloads together. Enable it to get a single-file download.
Note that the option to stitch multipart videos together requires Permute 2.3 or later installed.
Examples of sites that do this: