> They have no real reason to dislike her.
Imagine you're a freshman in college. You join a club and make friends with the people there. Quickly, the club becomes the center of your social life as you make friends with other people there and most of your time, formally or informally, revolves around the club.
As you get older, the club changes as old members leave and new members come. In two years, you feel like things are "going downhill", but just try to do your part a little better, and while you don't feel like you can have any effect on the larger portions (like the big introductory events) you have a little social circle that enjoys what you do, and you focus on making that the best place it can be.
However, when you're a junior, the leadership of the club starts changing rapidly. In six months there are two club presidents, each less liked than the last. Because of the way the university is structured, it would be very difficult to start a new club, or to get members of the previous club to come to the new club, and you've built up a lot of credential within the organization that will evaporate if you leave.
You're in a difficult situation now. You've invested a lot in this community. But top-down changes, possibly combined with normal drift over time, make it look like this community is going downhill. Suddenly, you feel like your little social circle will become an island in a hostile place rather than a part of a bigger organism.
>As a casual user I have no need to dislike Ellen
A casual X of any Y has no real reason to feel strongly about changes in Y. If I casually played golf, I wouldn't care if the rules changed. If I casually wrote iOS apps, I wouldn't care if Apple took a bigger or smaller cut. If I casually participated in politics, I would be unconcerned about changes that people who are above the level of casual are quite concerned with.
Deciding that because you do not feel personal investment with communities on reddit you are somehow above the "children" who do is misinformed. If you are a casual user you have no reason to be invested and no reason to care.
But to someone who feels close to a community that happens to live on reddit, these changes are scary. All of a sudden the administration team of reddit isn't the friendly, startup-vibe-having, tight-knight team of nerds that do cool april fools pranks and sometimes leave witty comments in pun threads. The Reddit admins have become depersonalized, and lately, they have been making changes that are very intrusive into the site. Whether you agree with those changes is immaterial. The fact is Reddit has moved from a mostly hands-off admin stance to a more hands-on one.
It's impossible to summarize this in a straight answer, because the fundamental reason why people are so upset over this (you are right that they are minor events -- only in the context of the larger reddit/community relationship do they become significant) is mere fear of change, and of the unknown. I think Redditors are very aware of the precarity of their communities. They don't want to lose them, but they think they might now, more so than they used to.
I hope this gives you a better idea of why Redditors might be upset. It's important to remember that every time we can't think of a likely true motivation for someone's actions, that is our own ignorance of human psychology, not a signifier of irrationality on the part of the person we observe. It's easy to write this off with bigotry or ageism, but that doesn't bring us closer to the truth.