What caused the "neutral" accent? What caused the Maine accent? What caused any other accent? Accent is a matter of perception. People who speak differently from you have accents. However, Southerners are generally aware that they are perceived as having an accent, and do tend to refer to people who don't speak the same way as being "accentless", unless they too have a noticeably different way of speaking, like North Dakotans, or New Yorkers, or Eastern Massachusetts.

Now, as for what caused the Southern accent, I would have to know which Southern accent you're talking about, because there are many and they are derived from different origins. North Carolina accents are far out and away derived predominantly from the Scotch-Irish, which is another term for the Ulster Scots, who were the largest group of settlers in North Carolina. If you ask any North Carolinian what their ancestry is, the answer you usually get is "Scotch-Irish and a little Cherokee". Out by the coast you have more lasting influence from English colonists. Tidewater Virginia, and the port cities like Charleston and Savannah (that "moonlight magnolia drawl" that is so stereotypical of old Southern accents, but has actually never been found in NC) have a great deal of influence from England, which contributes to the lack of "r"-sounds---as upper class Londonite accents became non-rhotic (dropped r's after vowels), the upper class in these cities (and also Boston and New York, with different results) began dropping theirs as well, so that they would stay fashionable.

Basically, though, I'd need more information to answer this, and this question will probably get a far more in-depth and technical answer over at /r/linguistics.

Citrusonic has very deftly answered the question for you, but I would like to just discuss an issue with the question itself. Your question reveals that you are making an incorrect assumption about language (don't worry it is an incredibly common one for people that haven't studied linguistics or the history of langue).

You seem to be assuming that their is a "proper language" and then accents arise out of this. This is not how languages work in reality. What is considered the correct accent of English, for example, changes based on where you are in the world and what date in history you are looking at. Everyone has an accent to someone else and no one accent is correct.

As to why the southern accent sounds "strong" to you, it is simply based on how much it deviates from the accent you are born with and the fact that you probably haven't had a lot of exposure to the accent. Southern accents don't seem strong to me, but someone from the rural northeast sounds very very accented to me.

Edit: source John McWhorter Word on the Street: Debunking the Myth of "Pure" Standard English ISBN 0-7382-0446-3