It's not random. Clients generally download the rarest pieces first. That way, the torrent is less likely to break when the swarm disappears. If everyone downl

It's not random. Clients generally download the rarest pieces first. That way, the torrent is less likely to break when the swarm disappears. If everyone downloads the most popular pieces first, then as the swarm shrinks, clients will soon only have pieces that everyone else has and nobody's downloads would finish.