When did the r disappear from Southern speech, and how did it come to disappear?
The custom of dropping it was not borrowed from the North, nor inherited from England.

Many Southerners--most Southerners-- put a y into occasional words that begin with the k sound.

For instance, they say Mr.K'yahtah (Carter) and speak of playing k'yahds or of riding in the k'yahs.

And they have the pleasant custom--long ago fallen into decay in the North--of frequently employing the respectful 'Sir.' Instead of the curt Yes, and the abrupt No, they say 'Yes, Suh', 'No, Suh.' But there are some infelicities.

Such as 'like' for 'as,' and the addition of an 'at' where it isn't needed.