No seriously, how does the money supply decrease?
In the normal course of events, money is created (magically brought into being) by borrowing it from a bank. It is destroyed by being repaid to the bank. It therefore should be your civic duty to borrow as much money as possible,
and never pay it back. In the current regime, who knows what is going on.
I wonder if all that stimulus money and the furlough payments were used to pay mortgages and bank loans, which would destroy the money and be deflationary, which would be A Very Bad Thing. I also wonder if the central banks globally have decided to reign in the QE etc, which will have a pretty immediate and intentional effect on the various stock markets, for their beloved "October suprise". Luckily the people in charge are jolly clever and know exactly what they are doing, so no need to panic.
In answer to your question - I don't know. May be interesting to find out out the mechanism. I shall look further.
Edit: from the same chart source, M1 actually went up by a spooky 6,666 million pounds, whereas M2 and M3 both dropped (by 897 and 19,665 GBP respectively). Does this mean that people have been converting savings into cash? M1 is cash and deposits only; M2 is M1 plus "near-cash" equivalents, such as savings accounts etc, and M3 is M2 plus even more esoteric stuff (like the bizarre shadow banking system, so who really knows...)
Perhaps the question is actually "What is money"?