I wish YT would actually identify WHY they flagged the video and stated it on a page in detail, so at least then we could fix the issues or prevent them from occurring in the first place!
I didn't have any issues with my videos getting demonetised until about 3 weeks ago, but since the every single one has been hit ! I have been uploading them, fixing the demonetised status (asking for manual review usually, but sometimes changing tags fixes it), and once it is monetised I then make it public.
I don't earn much money at all from my channel as it is nowhere near the size of Dave's, but I am sure the hit to the income is pretty hard in Dave's case, as also stated by Dave and others, the first day or two are the most important as that is where most of the money is earned from the video, in my case it costs a couple of $, but when you are talking tens of thousands of views being lost, that is likely to be in the hundreds of $ range.