Why did Axl Rose sing for AC/DC?

I'm curious, could you elaborate on why Axl Rose, the lead singer of Guns N' Roses, took the stage to sing for AC/DC? It's not a usual occurrence for a lead singer of one band to perform with another band, especially one as iconic as AC/DC. Was there a specific reason or occasion that led to this collaboration? Did AC/DC's lead singer, Brian Johnson, have any health issues or other reasons that prevented him from performing at the time? I'd love to hear more about the backstory behind this unique event.