I used to work in a call center for a major hotel/travel company. There are plenty of reasons why something like this happens. I'd bet that Mary2e is correct. It's not really scheduled system maintenance. Rather, it's an unexpected outage and they simple use the maintenance message instead of saying "something broke and we're scrambling to fix it".

Although, there are also times when nothing breaks and an entire call center gets knocked offline. For example, a power outage due to severe weather. I also remember a time when the number of calls in our queue doubled instantaneously. Later, we heard that one of the other call centers was evacuated due to a fire alarm.