Possibly

The "Monday volatility" is, in my opinion, an example for the weird correlation between Bitcoin and US stock markets. We're just now experiencing another miniature Trump Dump, although this time Bitcoin's reaction is even less pronounced than the stock markets', but it is there.

There would be almost no reason for increased volatility on Mondays if it wasn't for markets opening which were closed during the weekend, but I have observed it several times that a tendency change occurred after the US markets opened. (I've however not found stats for that. If someone knows 1-day volatility charts for BTC it would be helpful to confirm this idea, I only know the 30/60 day volatility charts from Bitbo ...)

My best bet is that it is a self fulfilling prophecy: Traders have noticed that often Bitcoin followed the stock price in the past 4 years, and thus they tend to sell if the US markets open with falling stocks, and to buy if the stock markets' tendency is bullish. And thus they're reinforcing this correlation - and as it "seems to work", they continue to do it, even without Bitcoin-relevant independent variables being present.

But this only works if there are no "internal" Bitcoin events which "decouple" the Bitcoin price from other markets' influence due to their own dynamics, of course (China mining ban, ETF approval, Terra/Luna, FTX, Segwit/Lightning, MtGox in 2013/14, and so on).