What made Rivkah partially slide off the camel when she saw Yitzchak?
Rashi: She was awestricken by Yitzchak's awesome appearance. 1
Seforno: She inclined her head out of respect for Yitzchak.
Rashbam: She was riding full-saddle (like a man) - not side-saddle, like a woman (out of fear, due to the height of the camel), so she moved to a more Tzeni'us position.
Ramban (citing the Ibn Ezra): The Pesukim are inverted, and she slid off the camel after Eliezer, in reply to her query (in the following Pasuk) told her who 'the man' was.
Ramban: She sensed that the man coming towards them was coming to greet them or to invite them to come and stay with him, so she acted modestly in the way of women, and she turned away from him.
According to the Ramban's version of Rashi, she did so out of shyness/embarrassment.
רש"י: ראתה אותו הדור: למה פחדה ממנו?
גור אריה: מפני שהיה הדוּר חשבה שהוא שר או חשוב.