Covid stress alone not a sufficient explanation as other nations with lockdowns and economic insecurity did not have similar numbers. This was an American thing.
Violence by police resulting in BLM protests and together resulting in poorer utilization of and cooperation with policing a hypothesis, but if so why not increases in other crimes?
My guess is that the biggest issue was the lack of school and other social structures for those most at risk for violence both as victims and as perpetrators: teen males. Other countries kept schools open more. We closed not only schools but also play areas, lake fronts, pretty much anything would keep youth occupied. At risk teens at higher than usual stress levels often did not sign in to classes and often had caregivers who were frontline workers unable to supervise them constantly. Structure and support were both absent.