If I know about the change in advance, then I can prepare myself for it and plan a new routine or schedule for me to follow, so the change of routine doesn't bother me too much. However, if the routine is interrupted or changed in little or no advance, then I find it more difficult to deal with. It interferes with my ability to function and upsets me or makes me nervous, because I don't get a chance to prepare myself for it or come up with a new plan/routine/schedule.
For example, if my morning routine is interrupted, for example by someone insisting that we leave the house 10 minutes earlier than we usually do so that we can stop somewhere on the way to work, it messes up my whole routine and makes me feel rushed and I'll function worse than normally, for example forgetting half of the things I needed to do or take with me and I'll feel stressed about this for a couple of hours. Or like happened recently, we invited some people over for lunch and asked them to come at 12:00. They called at 11:30 to say that they would come around 12:30, and then they finally showed up at 13:05. That upset me because it interrupted my cooking routine and I had to deal with constant changes that came with too short notice, which made me nervous, so the whole thing threw me off.