Last week's volatility came with an unsual twist: lots and lots of volume.

It's not a hard and fast rule, but volatility often comes with low volume. Think of traders as skiers on a slope: in the morning, while most of the resort is working off last night apres-ski, the early birds can zig zag about like crazy. In the afternoon, when the piste is full of people, you're pretty much forced to make slow, easy turns.

Likewise in the markets, usually. But last week saw massive amounts of trading as the markets ricocheted. Last Monday 9.7 billion shares traded as the market fell away. On Wednesday: 8.2 billion shares. The daily average for last week was 7.3 billion shares. But average daily volume for the year so far has been a paltry 4.26 billion shares.

I spoke to a lot of people on Wall Sreet last week, and they all had the same mantra "a lot of guys are out." Meaning, either on vacation, or out of the market, sitting on the sidelines, waiting for things to settle down.

So who was trading? The machines, that's who. The Wall Street Journal reports today that high-frequency traders have roughly tripled their stock trades this month. Tabb Group, a markets-research firm in New York, told the Journal high frequency traders' share of overall U.S. stock trading volume rose to about 65%, up from about 53% during the months before the August turmoil. Last Monday high-frequency traders made record profits of about $60 million in U.S. stock markets alone, Tabb estimates.

All this mechanical activity is a contrast to the flash crash of May 2010, when many high-frequency traders stepped out of the market for a period, only stepping back in after the makets had plunged. This time round, most kept trading, and even increased their activity.

Follow Paddy Hirsch at @paddyhirsch