Netziv(Bereishis 24:64): And Rivkah lifted her eyes – and she saw Yitzchok while he was still praying and he was at that moment like an angel of G‑d – extremely awesome. and she fell off her camel – Because of her great fear and awe. However she did not know who she was afraid of. If she had been sitting with Eliezer the servant on the same camel and she had been sitting behind him then she would have relaxed when she would have seen the man go and greet the servant and seen them talking together like all men. Then eventually when she would have been informed who it was – her fear would already have dissipated. But since she was sitting together with Eliezer she didn’t wait – but asked immediately who it was out of great fear. Who is that man - that makes me agitated and frightened? We see in Bereishis Rabbah that the term “that” implies someone fearsome and frightening. Therefore when she heard that it was her husband she took out her veil and covered her face out of her great fear and embarrassment because she realized she wasn’t fit to be his wife. From that moment on that fear was permanently planted in her heart and she was not able to have the relationship with Yitzchok that Sarah had with Avraham and Rachel had with Yaakov. In particular when the others objected to something their husbands had done they were not embarrassed to speak firmly with them. Rivkah was different. All of this served as a necessary preliminary to the events that would follow in Parshas Toldos when Yitzchok and Rivkah had strongly different views. Nonetheless Rivkah could not find the courage to stand up to Yitzchok and defend her views – even though it was true that Esav was a fraud. The same thing happened at the time of giving the berachos. In fact all of this was the means by which G‑d caused the berachos to be given to Yaakov in this manner as we will explain later. All was done with Divine Providence from the beginning - that Rivkah should arrive when Yitzchok was praying and thus she should see him as an awesome and frightening spectacle – it all happened according to G‑d’s Will.