Eh I don't thinks there's actually any explanation as to why they revere the Dragonborn. As far as I know the original Dragonguard just immediately knelt upon hearing Remans proclaimed him Dragonborn and swore fealty to him and then went on to help him conquer the rest of Tamriel. Seems kind of odd considering they still went about their Dragon genocide well serving the Dragonborn up until sometime during the Second Era.
Hell apparently there entire reason for Invading Tamriel was to continue hunting Dragons after wiping them out in Akavir. Which if true makes me wonder how they knew that there were tons of Dragons in Tamriel as well. I've always liked that Atmora was/is Kamal. The Dragon cult in skyrim does seem to have a lot of similarities to what little we know of Akavir. Like the Dragon scale armour from Morrowind looks a lot like the Dragon Priest robes.
But back to the point no clue what so ever why they would revere or serve a Dragon in a mortal body. Maybe because they're the ultimate dragon slayer like Delphine says? They no doubt had Dragonborn in Akavir to help in their war with the dragons over there.