Facts: AA passenger flying A (SEA) to B (DFW) then passenger connecting on AA from B (DFW) to C (TUL).

A to B flight was known to be running an hour and a half late when the aircraft began flying from B to the A for the A passenger pickup. Because of this A to B delay the traveler would miss his scheduled B to C flight.

To keep that from happening the AA computer automatically rebooked the B to C leg to a later flight which would allow the delayed passenger to have plenty of time to make the just rescheduled B to C later flight. So far so good.

BUT the AA computer also rebooked the A to B segment to a later flight as well creating a needless tighter connection to the rescheduled B to C flight.

Why?