On aggregate, large shops like Virtu are involved in market making strategies. There's various classes of market making strategies, and it is unnecessary to distinguish further here for the purpose of answering your question. For your curiosity however, Virtu is especially known for pure arb market making strategies.
Without diving into technical explanation, the easiest economic intuition to understand is that a market maker is acting as a price intermediator and looks to hold on to risk temporarily for their "customers" and to transfer risk to its natural sink. (Note: A market maker generally doesn't actually have a direct customer relationship but the market matches them to other participants in what amounts to a provider-client relationship effectively).
There is significant volatility on a day like Aug 24, 2015, which means significant pricing uncertainty. If there is greater pricing uncertainty, you can guess that there is greater "customer" demand for price intermediation.