Why did they suddenly decide to ask Rivkah for her opinion?
Rashi: Because one does not marry off a woman without first obtaining her consent. 1
Rashbam 2 : In spite of Eliezer's reply, it was simple etiquette to ask Rivkah herself whether she would not prefer to wait, in compliance with their suggestion, and to leave in another year or so - even to travel with someone else if need be.
Malbim: They thought to delay through her. They asked if she will go with the slave; they assumed that she will want her family to accompany her; this will be an excuse to delay.