Some of the imgur videos or accounts are private and some videos are geographically restricted by the owner. We cannot access these imgur videos, thus they cannot be downloaded.
When you left click on the download link it is normal the browser starts to play imgur video. You need to right click on download link and choose "save as" to save the imgur video.
On a PC, they are usually saved under "Downloads" folder. On IOS and android devices you can save imgur videos to browser download folder or directly to Photos app.
Most of the imgur videos we serve are in HD or Full HD format. This means that you can download highest quality imgur videos including 1080p imgur videos and 720p imgur videos.