This is likely the result of two things: competition and scale.
Schwab recently lowered the expenses ratios on its index funds to as low as .03%, and Vanguard had to keep up. Arguably, the Schwab funds are better because there is only a $1 minimum to get the .03% ER, unlike Vanguard's $10,000 minimum for Admiral shares.
The fund will not necessarily lose money on the reduction in ER. Vanguard has fixed costs and incremental costs. As more money is invested into the fund, the fixed costs as a percentage of total costs go down. I don't know, but I would expect these fixed costs--think websites, fund managers' salaries, advertising--to be the majority of the total expects. I would expect that incremental costs such as mailing statements, customer service reps and trading costs to be small. As more money is invested, these items don't cost VG more, so the ER can fall.