Because they trembled, meaning that they became filled with fear and awe of Hashem, therefore they stood from afar, which is the aspect of “hearing.”
In other words, hearing is a result of distance, which can either cause fear, or can be the result of fear. Because someone is afraid of something, therefore he moves away from it so that he only hears it, and cannot see it. Conversely, something that you feel distant from because you can only hear it and not see it, will cause a sense of fear towards that thing.
Because the Jewish People were afraid of Hashem, they moved away to only hear His message from afar, through Moshe Rabeinu, instead of directly from Hashem. Because a person realizes how distant he is from Hashem, and he only hears Hashem’s message but cannot see Him, this causes the person to fear Hashem.
This is connected to the idea (Sefer Yetzirah 1:8, Tikunei Zohar 7a) “if your heart runs in yearning for Hashem, then afterwards return to reveal Hashem’s Oneness in this world through Torah and Mitzvos.”
The Mittler Rebbe explains (Maamarei Admur HaEmtzai, Bereishis, page 504 and 544): This idea of “returning” means to feel embarrassed of Hashem’s greatness to stand before Him and want His closeness. This comes from contemplation of Hashem’s greatness and contrasting that with his own relative lowliness.
This feeling of embarrassment before Hashem will motivate the person to simply fulfill Hashem’s commands out of respect for Hashem’s greatness and majesty, without excepting anything in return, like experiencing a feeling of closeness. This is like when a regular person hears about the greatness of the king but never sees him. He is filled with awe for the king and fulfills his command without expecting the king to repay him with a private invitation to his palace.
