Being called to work an additional shift is only one possibility.
The boss might also be calling, e.g., to find out how to convert a Microsoft Word file into a PDF... or because he needs you to explain to him how to create a Table of Contents... or because he can't figure out how to turn off the spellcheck, or create a header in a PowerPoint, etc.
I had a boss like that once... Was the most-inept fellow with software you could imagine... And was always very apologetic... "Really, it will take only a minute!"... But, of course, it took much longer than a minute...
Regards,
Even though I was an engineer in a scientific billet I was the most computer literate of all my coworkers. trying to get a computer retard through the steps to do almost anything, even if it was really simple, would take me an hour or more while on the phone.
Having to describe what "right click" meant would take two minutes or more before they would comprehend.