It is often very complicated. Sometimes it is computer trading by MM computers. Other times it is a tactic by buyers and sellers to encourage buying at the ask and selling at the bid "IN VOLUME". Many stock watchers (investors) do not see the trade by trade data or the L-2 data all they see is the closing price at days end or a spot price 20 minutes delayed of the last trade and they may act on that data!!! Also sometimes a buy at the ask or sell at the bid in tiny volume may trip an ill advised stop loss or buy if, or sell if order. Sometimes an MM computer may see the chance to trip an if order by doing a so called paint job.