Just because stock values are determined not based on economic data, depressionary trends, or inevitable sovereign defaults, but just purely on what time of the day it is. Note the beautiful and not all that subtle explosion in the AUDJPY at precisely 3:30pm Eastern (the key gating factor for stock ramps), a time which has long been the all clear signal for the robots to go apeshit. So yes - at 3:29pm stocks have value X, and at 3:31pm it is 0.5% higher, just because RenTec's thousands of teraflop cores said so.
Tick chart for the AUDJPY shown below, with the 3:30pm boundary highlighted in red.
Update: to all suckers who took the bait: better luck next time.
Ahh, efficient markets.