liquidity. it drives mass selloffs. the day is down a bit. as the end of the day nears, people begin to tally their perspective losses if nothing changes. a lot of the trader are leveraged. they're trading with borrowed money, and they've just lost a shit ton of it. but there are some stocks that got through the day alright. these leveraged guys aren't down on those, so they sell them to pay what they owe on the stocks they got killed on. then those stocks begin to fall too. pretty soon everybody is selling everything to get cash to pay the bills they owe. it turns into a bloodbath.  yeah, it's turning paper value into cash.
Canada