this story confirms what I've been sensing for some time: I'm increasingly getting this kind of excuse/explanation even from good, responsible students, because it's no longer possible in many jobs to arrange your work schedule more than a week or two, let alone a semester, in advance.
Even if managers (some of whom are my more responsible students, who have worked their way up after 4-6 years in the business) are sympathetic to the difficulties of juggling classes and work, there's very strong pressure from corporate headquarters to hire and retain only workers who are more or less willing to be on call, and to schedule them only when and if they're absolutely sure to be fully occupied.
Add colleges' similar practice of opening up new sections only when and if there are enough students on the wait lists of other sections (not necessarily scheduled at the same time) available to fill them, and I don't blame students for feeling like they're in the middle of a tug of war.
On a related note, several friends' college-age kids have had no luck this summer finding local jobs, because employers want workers who are available year-'round (and, undoubtedly, on-call as well).
Well, at least if they go on to earn Ph.D.s, the adjunct market will feel familiar.