Jan 2002 - halliburton and oil index join

The stock price of halliburton (HAL) and the general oil index (XOI) were independent stock. Halliburton trades roughly at 25 million shares as it's daily volume. then, around the new year of 2002, the stock plummeted. Then there was a single day when the volume traded lept to almost 300 million. within a couple days volume had returned to the avg. 25 million. Ever since that day of massive volume, the price of Halliburton and the Oil Index have been linked down to truly minute price variations. Seemingly to the penny. it's bizarre to see the charts overlayed. So, wtf? I would have said "the war" but the massive volume that day makes me go hmmm... anyone knowledgeable about this stuff?