What are the motivations for publishing new editions of an existing textbook, beyond new discoveries in a field? In many of my undergraduate classes, professors required up to date editions of textbooks. This was across a number of subjects from Computer Science to Accounting and across introductory and more intermediate levels.

What is motivating new editions of textbooks? Are professors who published contractually obligated to publish new editions? Are they incorporating student feedback?
Publishers want new editions so that they can make money selling copies of the new edition and reduce the market for used copies.  The new edition might be significantly updated, but in many cases the updates are small.  For textbooks in lower division general education courses, new editions come out as often as every three years.  

It's quite common for textbook publishing contracts to include clauses that give the authors right of first refusal to produce an updated edition but allow the publisher to add a new coauthor and produce a new edition if the authors are unwilling to do so.