Since the start of the canon series, Uatu has been there, narrating, watching the different alternate realities, showing that there is more to the multiverse than what we already know, only once did he interact with a character, but in Marvel's What If..? Episode 8, Ultron having all the six Infinity Stones had to push him to his boundaries, who convinced him to start to intervene and break his oath?
Who Convinced Uatu to Intervene in Marvel's What If..? Episode 8
We first saw Uatu telling the story of how Peggy Carter became the First Avenger instead of Steve Rogers and he continued to tell us stories of alternate realities of the ones we know but with a different circumstance. Never was he the one to initiate an interaction with the characters, however, Doctor Strange became aware of his existence in Episode 4 but still, he did not prevent such universe to collapse.
Despite having one universe collapse and become the prison of Strange, Uatu did not stop it from happening even when he is capable of preventing it to happen. However, when the whole multiverse is in danger, Uatu tried his very best not to intervene, even up to the last moment, he left hope to those who live in the universe.
Even when Nat and Clint were almost giving up, Uatu was just watching, like his very oath of being The Watcher and of non-intervention to the natural order of things. For years, Uatu has been true to what he promised, of watching what happens and not getting in the way of how things are supposed to happen.
Related: Here's the Voice Cast for Marvel's What If...? Episode 8
However, in What If..? Episode 8, What If.. Ultron Won?, the evil AI got all the six Infinity stones and from then, he became one of the most powerful villains that he became aware that there is more to the universe he destroyed, that there is a multiverse out there waiting for him to put some peace and quiet, a.k.a. killing all those in existence.
Ultron got ahold of where Uatu was watching all the alternate realities, giving the evil AI access to whichever reality he would want to destroy. The Watcher went into hiding and there, a pivotal character convinced him to finally break his oath, one who did the same for the one he loved most. Doctor Strange Supreme, the evil version of the sorcerer, convinced Uatu to break his oath just like he did his.
In Episode 4, Strange lost Christine, the woman he loved most, his own heart. He swore an oath to protect the Time Stone and broke it along with all the rules that come with it just to bring Christine back. Because of that, his universe fell apart and became his prison. That is the same pocket universe where Uatu went into hiding.
Just like Strange, Uatu would have to break his oath, but this time, in his case, the whole multiverse lies in his hands. If he could stop Ultron from breaking his oath, it might really pay off.
All episodes of Marvel's What If...? Season 1 are available to stream on Disney+.