YouTube usually takes a while to categorize videos. This is because YouTube develops data about what your video is and who is watching after it has been posted a few days.
From time to time, there are anomalies with YouTube - videos being together that share no singularities - but this usually only last a few days at the most.
There are a couple things you can do to help YouTube: 1) make sure your video is titled and tagged correctly 2) provide a detailed description.
