I recently noticed that one of my three Agave 'Blue Glow' was rotting away. Dark soft tissue was radiating upwards into yellowing leaves. My main fear is the Agave Snout Weevil of which I've heard such extreme horror stories.
Digging up the plant, I didn't spot any adult weevils nor any of the larvae that many online sources said I should see. And though watered the same morning it was excavated, the soil didn't seem to be too wet. The other two nearby Agaves seem fine (for now). So I'm not sure what is the cause - but I think I'll be treating the area just to be safe.