As great as they are, computers can't account for every single situation. Software sometimes has restrictions that still need humans to override. In the olden days, this entire process was done without auto-rebooking. Everyone had to talk to a Delta representative (or their
travel agent). What Delta has today is more efficient and only a small number of passengers find themselves with broken itineraries.
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Technical Explanation (aka More Information than most Travelers Need): As I understand it, Delta's VIPER software is designed to be revenue neutral. Every ticket belongs to a fare class designated by a letter. For example, a discounted economy ticket might be fare class S. The actual fare (e.g. $150 or $250) can vary within a fare class. When VIPER runs, I believe it is only allowed to rebook passengers within the same fare class. If nothing is available, it assigns an "impossible" itinerary and the ticket gets placed into the queue for a human being to fix. Keep in mind that on a multi-leg ticket, the same fare class would need to be available for all flight segments.
Please note that this restriction is eliminated during periods of irregular operations (IROPS). If a flight gets canceled on the day of travel or due to a waiver, pretty much any available seat (in the same cabin class) is available. In the event of a same-day cancelation, the easiest way to rebook yourself is via the website. What you see online should be the same as what a Delta representative can see. That's materially different from an itinerary change that happens days/weeks/months in advance.