Yes, A number of Fed ex employees probably volunteered to work on Christmas day. This is especially true if they were being paid more to work on this day. Many people may not have had big plans for Christmas day and may have wanted to work to make some extra money.
I'm sure there are some people who volunteered for overtime, but I don't think the majority probably were told they had to work. I think Fed Ex should not have had people work on Christmas Day. Christmas Day is a holiday meant to be spent with family and friends. I doubt all of the employees volunteered.
I've worked in industries where holiday work occurs and I think it's likely they paid extra, but I think the workers were still essentially forced. It's also possible that they asked select workers to do so and they felt that they had no option but to say yes. I've seen that plenty of times.
I would highly doubt that all, or even the majority, of Fed Ex's workers volunteered to work on Christmas Day. I'm sure that there are a few employees that did; either those who do not celebrate Christmas or maybe those who desperately need the overtime money. But Christmas is a major holiday and I would think that the great majority of workers would not want to work on Christmas.