Just as Chris described, AGU has a system for organizing the meeting. There is about a third of the meeting set aside for special sessions, whose themes are selected before abstracts are accepted. The special sessions each have a pair of chairpeople, who can select which of the abstracts submitted to their sessions are placed in talks and which should be posters, and which are more appropriate for other session. Each section, like seismology or tectonophysics, also has a pair of overall meeting organizers, who do the rest of the abstracts, as well as special sessions whose organizers are not inclined to set up their own sessions.

Probably more detail than you wanted to hear, but consideration is given to a sensible sequence of talks, keeping things interesting, politics, mixing up the gender/seniority of sessions. Plus a degree of randomness as rooms assignments prove difficult, people don't show up, equipment malfunctions, posters are misnumbered, etc..

John