EDIT: My theory is probably wrong, but it's an interesting observation, and it would be good to see a chart that takes into account the available BTC participating in volume for such charts
It's important to remember the value required to create volume in Bitcoin exchanges.
If Bitcoin is valued at $10, then $100 can move 10BTC, but if Bitcoin is valued at $100, then $100 will only move 1BTC.
On Bitcoincharts.com, at least, if you view all data, click the checkbox on the right to show "Volume in Currency"
That will illustrate the difference.