1 Copyrighted Material(Number 1 CULPRIT) **If you did not record the video, it belongs to someone else! Even capturing your screen may violate someone else's copyright...
2 Your COMPETITORS flagged your video on purpose using many accounts. This happens A LOT in the "Make Money Online", "Lose Weight" and "Adult Dating"niche/market/videos. Some people just hate their lives so much they have to ruin other people's lives as well
You MIGHT NOT ALREADY know this....there are Softwares out there that are used SPECIFICALLY to flag your video(s). Remember, anything a User can do... A SOFTWARE/BOT can do. The process can be fully-automated.
3 Your video is VERY Short and SPAMMY. For ex: under 60 secs, provides no useful content with just affiliate links. These kinds of videos get removed very easily without much flagging!
4 And many other factors/reasons. (MAIN 3 Reasons ABOVE)