We are not yet clear what is causing this, but here is a temporary workaround you can use which makes use of exported PNGs: Generating High Quality Animated GIFs using Imagemagick
Also, if you export as MP4, you can drag and drop your video into a tool like Gifrocket or ‘Drop to GIF’
We have heard more reports of this occurring on 10.9.5, so if you are able to upgrade to the latest version of Mac OS that will likely fix the issue for you.