There are a number of possibilities, any of which could eventually separate this emulsion:
1) time. Left long enough, this dressing, even with the mustard will separate.
2) temperature. If you put it in too cold a refrigerator, (closer to freezing), it's splitsville.
3) over-agitation. Too vigorous a stirring and the unfiying properties of mixing will backfire.
4) adding the oil too quickly at the beginning will form an unstable emulsion.
5) adding all of the acid at the beginning will also form an unstable emulsion.
See my article on emulsions here on Cheftalk:
http://www.cheftalk.com/content/disp...2&type=article
Mark