The Lancet has made one of the biggest retractions in modern history. How could this happen?

The now retracted paper halted hydroxychloroquine trials. Studies like this determine how people live or die tomorrow

The Lancet is one of the oldest and most respected medical journals in the world. Recently, they published an article on Covid patients receiving hydroxychloroquine with a dire conclusion: the drug increases heartbeat irregularities and decreases hospital survival rates. This result was treated as authoritative, and major drug trials were immediately halted - because why treat anyone with an unsafe drug?

Now, that Lancet study has been retracted, withdrawn from the literature entirely, at the request of three of its authors who can no longer vouch for the veracity of the primary data sources". Given the seriousness of the topic and the consequences of the paper, this is one of the most consequential retractions in modern history.