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.

I heard an educated gentleman say, 'Like the flag-officer did.' His cook or his butler would have said, 'Like the flag-officer done.' You hear gentlemen say, 'Where have you been at ?' And here is the aggravated form--heard a ragged street Arab say it to a comrade: 'I was a-ask'n' Tom whah you was a-sett'n' at.' The very elect carelessly say 'will' when they mean 'shall'; and many of them say, 'I didn't go to do it,' meaning 'I didn't mean to do it.' The Northern word 'guess'-- imported from England, where it used to be common, and now regarded by satirical Englishmen as a Yankee original--is but little used among Southerners.