Why are my watching hours reducing?
I think we need a bit more context around what you mean by "reducing" do you mean that they're lower this month compared to previous months? And the best way to find "why" hours are reducing, is to look at individual video watch time graphs to see what part of your video is "boring" to your audience.
This video did fairly well compared to my average video (still bad though. 20% retention is "horrible" in the opinion of most. You generally want at least 50% or more... but you can see my intro was boring to half the people so they dropped out... the rest of the video was "interesting" enough to keep most through, but people progressively got more and more bored, probably because my video was too long, and my face and voice is probably annoying. The topic at the time was good, so that's why it did well out of the gate.
This is one of my worst performing videos. It wasn't edited, the topic was pretty boring to most. I THOUGHT Halloween would be a good opportunity to draw some Halloween inspired dinosaurs, but evidently people didnt care, and my intro was boring, I probably talked too much by 2 minutes in and people just dropped out and didn't care. Maybe 3% of them skipped to the end to see if the video got any better.
Look at your watch graphs of each individual video, and be honest with yourself. Are you picking interesting topics that will interest your audience? Do you deliver your message clearly and quickly. Do you ramble on and drag things out longer than you need to?
All of these factors will affect your watch time. Then it's a combination of seasons. People spend less time on YouTube during the holiday seasons, so that's another reason why your watch time overall will decrease.