I've worked in store management before. We reduced hours/shifts when the owner/operators told us to reduce payroll hours. Also, sometimes with the seasons the amount of hours available change. And sometimes we would severely reduce the hours of someone when their performance wasn't optimal.
I would also ask why, in a curious way. If they are reducing hours for payroll reasons you can ask if your hours are going to go up again soon. If it is something in your performance you can genuinely work hard to improve. Feedback can be really hard to hear, but can help you grow a lot.