For the record, I'd like to correct a mistatement, "doin the butt" was a late 1980s dance.

Also, I meant to say that there are probably other R&B dances that are performed in a line which more closely resemble the steps that the Namibian women and men performed in those video clips. That said, the thrusting their hips to the side, thus emphasizing their butt does remind me of that "doin' the butt" dance. Unfortunately, I'm not a dance historian and can't come up with the past or present R&B dance or dances that the Amarula dance steps remind me of. Also, I wouldn't be surprised if those steps and movements are traditional steps that are found throughout Africa including in West Africa & Central Africa and elsewhere.

Although it's off-topic, for comparison's sake, here's a link to the song Da Butt, recorded in 1988 by EU {Experience Unlimited} from Spike Lee's movie "School Daze" of that same year:

http://www.youtube.com/watch?v=ZQUgFNEGmGI&feature=related
Da Butt - School Daze