>Research slated for publication in Organizational Behavior and Human Decision Processes shows that co-workers significantly overestimate how quickly senders expect replies to non-urgent emails -- especially those sent outside "normative" hours like nights or weekends.
The researchers call it "email urgency bias," a phenomenon caused at least in part by the fact that response speed has increasingly become a proxy for dedication and hard work. (The same way many managers see working long hours -- call it the "butts in seats" bias -- as a proxy for productivity.)
Another reason relates to closure. Sending that Saturday email gives you a sense of closure. You've thought outside the box. You've cracked the code. Boom: It's done.
Your head of shipping doesn't feel that sense of closure; in fact, receiving the email triggers her own desire for closure. Which means she has to respond.
The quicker the better, both because she thinks you expect it and also because then she, too, can try to put the matter aside until Monday.