Letterman house band performing with guest bands

[Khor1255]

I watched a clip of Screaming Trees tv debut and was shocked to see the band surrounded by various studio musicians and minus their drummer (who showed up at the end of the show to do a food eating joke Letterman was keen on. I'm wondering if anyone knows the details of how and why this strange situation was thrust upon various acts who almost to a man look highly uncomfortable doing their songs with extra members?