How did 'anyway' become 'anyways,' anyway?

All of the time I see people use these two words synonymously. For example:

Why did he move there anyway?

Versus:

Why did he move there anyways?

I always assumed that there was once just the word anyway that one day had an S added to the end for some reason. Is this assumption correct? What morphological process enables this to happen?

I've always thought the addition of the S was just an affectation. Slang usage.

People use laters/laterz online a lot in a similar way. There are other variants seen for anyhow, anyhoo is quite popular, spoken and written.

The 's' wasn't added; for some uses of the word anyways, it has always had an 's' on it. The OED calls it an adverbial genitive.

The adverbial genitive was a grammatical form in Middle English; to summarize, 's' was sometimes added to the end of a word to show that it was an adverb. We don't do that anymore, but some adverbs have 's' on their ends as a remnant of this, for example towards, forwards, besides, and always. Anyways is one such remnant. The OED has the first citation in 1560:

all those who are any ways afflicted ... in mind, body, or estate

Of course, anyway is not always used as an adverb:

Why did he move there anyways?

For this usage, it historically did not have an 's' on it. However, since the 's' is now meaningless, I suspect that people who add the 's' for the adverbial usage of anyways often also add the 's' when it is used as a conjunction. The OED labels this "dial. or illiterate".