Apologies for the long post. I don't know how much context you already understand, so I'm kind of starting from scratch. Please ignore anything that you already know. I'm not trying to talk down to you. I just wanted to cover all my bases.

In a capitalist society, it is useful for there to be a significantly lower paid portion of the population. This allows the extraction of wealth, and by extension, creates privilege.

There are many ways to do this, but one of them is to demonize an entire culture. That way, the average person will not feel the need or necessity to fight for their rights, you don't need every single person to hate this culture. You just need a good portion of the population. That way, it's controversial to defend them.

Since Mexico is border to the United States, there happens to be a lot of citizens coming in. That's pretty natural and not unexpected at all. But the alt-right has decided to demonize them because it makes them an easy target. There's also a lot of them and so demonizing that culture means that there is more underpaid people for the picking.

It also helps that they're brown. White people typically target brown people. And this isn't even unique to the United States.

Also, most people don't understand that that's what's going on. Most people are watching the news or hearing from their friends that the brown Mexican people are all dirty, and sending over their criminals, and are causing crimes and stuff like that. Statistically, it's the opposite. Immigrants tend to stay under the radar. Lay low. Not break the law. Not cause a fuss. That's one of the reasons that they're easily manipulated. Because they tend not to speak up when they're underpaid or are lacking benefits that they should have. This is even people who are here legally. There's absolutely nothing illegal about them being here, but they still have to lay low because... racism.

Why now? Probably because the elections are coming up. As it turns out, you really want your political rhetoric to get your base excited once every political election. The rest of the time, you usually can take advantage of them without too much fuss. It could also be that maybe your local area is just cycling through demonizing different groups, and it was time for the Mexicans to come up. But that makes it sound pre planned, and I don't believe in conspiracy theories. A few people might start the rhetoric on purpose, but then it becomes self-sustaining. Social media just tends to move through different outrage cycles.