Because there have been articles outlining the backing of his organizations to start this movement & march. Originally it started as a protest against the conditions in their own country -- being so bad they were going to march out of it. The intent at that point was to draw attention to crime, poverty and corruption within their own nation. Then this grew into a march to protest the Trump administration policies. There are multiple articles on the background of all of this including on CNN, etc of reporters embedded in with the marchers. Go read up - some interesting stories.
I don’t believe there is any credible evidence that George Soros is funding these migrants. I do however believe the hatred of George Soros is rooted in anti Semitic white nationalists that find their way into republican political propaganda. Take for instance Steve King, even Trump has promoted these sources. What I am telling you is that the caravan is not a Jew funded plot to dilute the white race in America.
So if we criticize Soros, it is automatically anti-Semitism? Kind of like any criticism of Obama was racist? Do you honestly think the criticism of Soros has the slightest thing to do with his religion, other than people pointing out the grim irony of a jew helping nazis round up people, which he did as a youth. I can promise you that if Jeb Bush or the Koch brothers were funding the stuff Soros funds, conservatives would be equally outraged at them. This despicable business of finding anti-Semitism and racism in any criticism of the left only encourages people like the Pittsburg shooter.
What if it was? Would you still support it? Of course you would. In fact, we know the left has been following a plan to bring in massive numbers of immigrants for precisely this reason. There have been numerous articles gloating about it, typically under the rubric of "demographics" dooming republicans. Call it the "California plan".