The pairing with Parashah Yitro, whose central episode is the revelation at Mount Sinai, thunder, lightning, a shofar sounding and the mountain itself trembling as the Ten Commandments are given, rests on the shared experience of overwhelming theophany: both accounts describe an encounter with the divine presence so intense that it shakes the physical setting around it, the mountain in the parashah and the Temple's foundations in the haftarah.
Where Sinai's revelation is given to an entire assembled nation at once, Isaiah's vision is given to a single individual commissioned specifically to carry a message back to that same nation, so that the haftarah offers, in miniature, the same structure the parashah establishes on a national scale: an encounter with God so overwhelming it demands a response, followed immediately by a commission to communicate what has been revealed to others who were not present for it.
Isaiah's own hesitation, declaring himself a man of unclean lips before he can accept the commission, echoes a theme found elsewhere in the Sinai narrative as well, the people's own fear of hearing God's voice directly and their request that Moses mediate the revelation for them, both texts recognizing that direct contact with the divine presence is something even a willing recipient approaches with genuine trepidation.