Fidelity wants to sell 100 million shares of their Cisco holdings in one shot. They don't want to wait for the quarter to be announced and are nervous about the whole market. They call their friendly broker and ask for help in finding a buyer or three. The broker makes a bunch of gentle inquiries to buyers with whom he regularly does business and who he knows have pockets deep enough to pay Fidelity the couple billion dollars they'll want in return for the shares. When buyers have been found and a price agreed to, the trade happens in one big fat block.