I also think they leave Mexico because of their instinct. And I think if they leave to early, It would be to cold for them to live in the U.S, and I think if they leave to late, they won't be able to make it to Mexico. I think the Monarchs knows when to leave because they canfeel the cold. If they leave too early it might not be warm yet. If they leave too late they will die because of the cold.
Monarchs have a sentes of what season it is.
When it is hot out the Monarchs will go North. I think they have been doing the same thing for so many tears they just know what to do. If they leave too early it will still be cold and they will die because of the coldness. If they leave too late it will be too hot and they will get burnt. Also there will be kids outside and they may kill them or keep them and not know what to feed them and when it has to be let go.