Well it's leaf spot, just don't know whether its bacterial or fungal - I suspect fungal, but not sure. Not much you can do about it, though you can try spraying with a fungicide that treats leaf spot - trouble is different organisms cause it, so its a bit hit and miss as to whether the treatment you buy will do anything. Yuccas are prone to leaf spot - feed the plant with Growmore raked into the soil at the base, keep it well watered during dry spells, repeat the feed in 6 weeks. Remove any leaf debris from around the plant, and remove badly affected leaves, as you've been doing. A sunny position in the garden helps to stop this happening - it won't kill the plant, though sometimes can indicate another, underlying problem with the plant, which is why I'm suggesting you give it a boost by feeding it.