In Season 3 of The Legend of Korra, fan-favorite villain Zaheer overcame his "earthly tether" and mastered flight, an ability that was previously only achieved by one person. Other airbenders with more training, like Avatar: The Last Airbender's Aang and even Monk Gyatso, were also able to soar through the skies, but only with the aid of their gliders or inside a sphere of air. Their inability to match Zaheer, a relative novice at the craft, seems confusing, but a new theory has suggested that despite their efforts, other airbenders were never able to truly let go of one particular "early tether."
An anonymous theory submitted to Tumblr fan account atlalore has proposed that the Air Nomads had all developed an unshakable bond with their flying bison, preventing themselves from accessing flight. They instead relied on the bison for this specific mode of travel that allowed them to reach their hidden temples in the mountains.
There are a number of context clues from both Korra and Avatar that lend support to this hypothesis. In the era before the Avatar, depicted in Season 2, Episode 8, "Beginnings, Part 2," Wan encounters a group of benders who have received the power of airbending from their Lion Turtle. They pick fruit from tall trees by floating on clouds, and later use the same ability to run away from Wan. Since these benders do not use gliders to fly, and seem to be able to hover in order to pick fruit, this skill seems more similar to Zaheer's than that of Aang or the other Nomads.
The clothing, forehead tattoos and preferred bending style of this group imply they would one day become the Air Nomads. However, when Wan visited their settlement, there was not a single sky bison to be seen, meaning the group had not yet formed a close bond with the animals. Therefore, it is possible that there is a connection between their ability to fly independently and the lack of bison.
In the following millennia, Guru Laghima was the only known person to achieve weightlessness, a skill so rare in Korra's time that most believed it to be a mere myth. Zaheer says that Laghima lived four thousand years ago, and the novel The Rise of Kyoshi states that Laghima's contemporary, Shoken, lived before the formation of the Four Nations. Seeing as he lived so long ago, it's possible that Laghima lived before every Air Nomad was assigned a bison as a child. If he was one of those who did not befriend a bison, this lack of a worldly bond could have been what allowed him to master weightlessness in the first place.
Similarly, Zaheer was likely the first airbender to develop his skills outside of Air Nomad culture in centuries. Never having a bison meant he wasn't held back by his connection to one. Of the most prominent airbenders seen in Avatar and Korra, Aang lost his people and culture but still had Appa, Tenzin had Oogi and Jinora had Pepper. All may have been highly accomplished, yet none spent any considerable length of time without finding a bison partner.
It is true that many non-Nomad Avatars also mastered airbending despite not having a flying bison partner, but they all came from cultures that did not try to renounce their worldly connections as the Nomads did. Additionally, the Avatar's role as the intermediary between the physical and spirit world essentially mandated that they maintain some connection to worldly matters.
The one shortcoming to this theory is its inability to explain why an Air Nomad would not be able to master flight after their bison had passed away. Zaheer was able to sever his final tether after the death of his lover P'Li, so it stands to reason that a similar scenario would have also freed any particularly talented airbender. It could be theorized that beyond their bison, the Nomads were also tethered to their culture and their fellow Air Nomads. Zaheer lacked any affiliation to begin with, making it easier to fully separate himself from others.
Regardless, this hypothesis does go a long way towards reconciling how Zaheer was able to master flight when generations of highly trained Air Nomads focused on untethering themselves could not. It also serves to highlight the importance of the symbiotic relationship that the Nomads shared with their flying bison partners.