Theory #1 - The 1st late arrival in Chicago resulted in the consist missing its' scheduled maintenance turn; which resulted in a late departure W/B. This effect snowballs into further late arrivals / departures. Lack of extra equipment for a make - up train compounds the situation.
Theory #2 - The W/B consist was previously another N/B or W/B train into Chicago, which is running late, leading to the situation mentioned in #1 above.
Theory #3 - Stuff happens --- Late trains get later --- The likelihood of a late arrival is directly proportional to how many people are waiting.
Not really that different from the current airline on - time situations regarding equipment utilization & maintenance scheduling.