The quasi-biennial oscillation (QBO) is a pattern of descending easterly and westerly winds in the equatorial stratosphere that repeats every 28 months on average. The QBO matters because it influences environmental variables and phenomena in both the stratosphere and troposphere, and because its recurrent nature offers the potential for predictability. However, in 2015/16 after at least 62 years of regular behavior the QBO experienced an unprecedented disruption. Here we report and examine a second and similar QBO disruption that occurred just four years later in 2019/20. This second event shows that the original disruption was not unique and suggests an increased chance of further QBO disruptions. The QBO splitting in 2019/20 was initiated by unusually strong southern hemisphere Rossby waves that dissipated momentum in the equatorial stratosphere. Large vertical differences in upwelling speed sustained the QBO split and contributed to an anomalous upward-moving westerly QBO regime.