Well, you're not mind-readers. If they didn't tell people they were doing something different this year, how were you supposed to know? Things tend to become ingrained if they stay the same for long enough. Easter at their house has become a habit.
On the other hand, I can sort of see where they're coming from, too. Hosting is a lot of work, and last year, when we wanted to do something different for the 4th of July, another family member got offended because we "always spend the 4th together". Maybe assuming was a mistake, but if they didn't let anybody know, who could blame you?
An apology would probably help, something along the lines of what other posters have said. It sounds like you didn't mean anything by it, and it was just a miscommunication.