It's not about statues, or really even about what they stand for. It is about white people who feel they are superior just because they're white, and they feel attacked for that (as they should). Their ancestors were willing to risk destroying this country over it, and they are no different. The various governments in the US have spent years and millions of dollars trying to lead us down the path of believing the Civil War was just a bit of unpleasantness that didn't go the way the South wanted it to, no big deal. Slaves were freed, so that took care of the issue. The educational system here was careful to continue that whitewashing, so that most people aren't even aware of how bad things have been. So generation after generation of white people have been carefully taught that since we don't own slaves anymore, that's all that matters. But the insidiousness of the message that whites are superior is embedded into every aspect of life here. THAT is the issue at hand.
Let me break this down into several pieces...
1. Whitewashing
History has most of the times been written by the winners, and their point of view. Why should the civil war here be any kind of different? "De bello gallico" from Caesar is the same, it has an century spanning tradition.
2. System inherent rassism
I guess we don't really need to discuss this further: the USA had an Apartheid system in place, before that slavery, practiced red lining and many of those old decisions are still a heavy burden to the affected people.
3. Political iconoclasm or pulling down statues
This is nothing new, and happens from time to time. Ironically most people are just ignorant to most statues; but pulling them out of the public though the people doing this do clearly show that they do believe that such statues are still having a substantial impact to modern society. If those people really would believe that statues have no impact there would be no reason to remove them.
This means that ironically the people who pulling off the statues are also the people who do believe the most in their impact. The term for this is "cancel culture", right? Deciding this in the end is a really tough cookie, the easier question would be probably who deserves statues in the end.
In short the USA are a very much divided nation with some heavy rifts, and that these tensions lead to such a movement was bound to happen sooner or later.