A set schedule means they have releases planned of ahead of time, but the reality of software development is that except for rare instances, they are nearly always flexible and will be adjusted as needed.

In terms of motivation, it could be for a ton of different reasons. Say a new feature was scheduled to be released next week, well if there were still significant bugs with it, then they will obviously not want to release it and adjust their timelines accordingly. If things are generally "good enough", they may just release it and patch them later. Alternatively, it could be something due simply to money. Maybe they don't want to do any overtime for something that requires a lot of extra work to make a deadline. Or maybe they want to release the feature in a different way or at a different time because it will make the company more money. Maybe they think releasing something might significantly hurt the game balance. Literally anything you can think of could be a possible reason depending on the issue at hand.