It takes something specific to create what might as well be termed a "golden horde." This isn't a matter of "dude got tired of the danger one day and a couple other people thought it was a good idea to leave too and suddenly 'we got us a convoy'".
I appreciate your approach but it's a grossly oversimplified explanation for what's happening.
For what it's worth, I've always been a proponent of closed borders except for a few vacationers and immigration through an education track with trade restricted to raw goods. I've held that view all my life and your not likely to change my mind on it though I'm willing to listen and consider. I'll also quickly admit that it is going to seriously bias my view and discussion with you.
Honestly I was against open borders until I worked with some illegals and my daily interactions with them changed my views. I felt it was very hypocritical of me to be saying "kick them all out" when I was spending 12+ hours with them some days. In my interactions, I saw some of the animosity that occurs between Mexicans and Central Americans. Most landscapers in Atlanta will tell you to be careful mixing the two groups and if you do, then make sure the crew leader can keep the peace. Some landscapers will only hire workers from one country in order to avoid any conflicts.
Honestly I think this convoy is either a group of "coyotes" who are being brazen or the work of a NGO. The groups were gathering a certain points and waiting for the convoy to pass though then they joined. It's too well organized to be random. It definitely isn't a "Forrest Gump" situation.
I saw a great post somewhere the other day that said "if immigration laws were changed so that they were no longer illegal immigrants, would you still be against their coming?"
I've worked and hung out with legal and illegal immigrants. Just like any other group, you have awesome people and horrible people. That doesn't really change the fact that those who didn't go through the immigration process legally are earning income illegally. It's a form of anarchy, disregarding laws because it doesn't suit you. When you increase the population that disregards laws because of convenience, it should not come as a surprise that eventually you will witness an increase in crime.
Now if you want to talk about changing immigration laws, hey, let's talk. Of course, you already know my position: the only way to gain citizenship is to go to a college, earn a degree (paying with their own money) in a set amount of time with a requirement to work at least one year in their field. If they already have an education in that field, it trains them to that country's method and work culture while introducing them to ideas and methods that they can use in combination with their own experience. It completely eliminates problems of supporting themselves, puts valuable people into the work force, and weeds out people who can't or won't do productive work while simultaneously increasing the general workforce ability and intelligence.
Or they don't want to live or work in a country that will treat them as a lower class of people. The Mexican people do not like Central Americans for the most part. Plus the US offers high wages and isn't engulfed in cartel wars like Mexico is. If you were fleeing gang violence would you want to settle in a country that has the same problems almost as bad as your home country, or would you seek asylum in a different country that didn't have problems like that?