Vanguard funds are owned by the investors in those funds. The fees that they charge are to cover the expenses, not make profit for a private company. If they're doing so well that the fees are brining in more money than they need, they lower the fees so that their owners (the investors in the funds) don't have to pay as much in fees.
Their business is doing so well that they can lower fees.
Vanguard has been reducing fees on a pretty regular basis.
My assumption is that the corporate leaders go for a dip in the McDuck money pond every year, and when the level is too high that it risks overflowing, they adjust the fees down.
Their business is doing so well that they can lower fees.