First of all the dress is worn that way, it slips down occasionally. Second of all she does pull it back up, and third of all, who cares if it slips a little?
The producers may have made her dress that way to make her seem more flirtatious. As I recall, wasn't Marty the one coming on to the news reporter guy?
I noticed this so-called slipup many years ago and after watching a few times I am convinced that it is her hand coming up that makes it look like her dress has fallen down as the wather can see skin. Well in my opinion that skin is just her hand and nothing at all went wrong. Before the dress is supposed to fall down her hand is already lifting up.