If it's that easy to upload a video from your camera roll to YouTube (Two clicks) it's not that hard to imagine that this can happen by mistake or by someone who doesn't know that it uploads as "public" by default.
Maybe they just wanted to send this video to a friend and didn't have the technical understanding that this will then be visible for everyone searching for it on YouTube.
Maybe they just wanted to send this video to a friend and didn't have the technical understanding that this will then be visible for everyone searching for it on YouTube.