I apologize if my question is too wordy or complex, but to clarify, I am talking about situations like, for example, Alaska breaking off from the United States to join Canada due to the Canadian population in Alaska.
I'm guessing that you're searching for something similar to Crimea and Russia.
If so, feel free to look into the history of Texas in the United States. If memory serves, Texas broke off from Mexico to gain independence, and later joined the United States. If memory serves once more, Texas attempted to join the United States not long after independence from Mexico. That's one example I can think of off the top of my head, hope it helps!
Edit: Oh, and reasons. English speakers in the Texas who had ties to the United States wanted to join the Union. White slaveholders moved to Texas en masse, and even though Mexico outlawed slavery in 1829, it neglected to enforce the ban in Texas. By 1835, Americans outnumbered Mexicans in Texas ten to one.
English-speaking whites objected to being taxed and complained of lack of military protection against Indians. But most of all, they sought to preempt enforcement of national laws, including the ban on slavery. In 1836, joined by several leading Mexicans in the region, they declared independence...Houston and the Texans expected the United States to annex the new independent nation, but the politics of slavery dictated a different response.
It would remain separate for nearly a decade before statehood, but not for lack of trying.
Source: David Henkin, Becoming America