could be some combo of higher humidity, particulate matter from brush fires, etc.
I hate light pollution with a passion, aside from the health issues associated with it (including higher rates of endocrine cancers), when I shoot astro, I use longer focal length lenses now.....astro in light pollution is junk between 24mm-80mm. I start at 150mm. That effectively eliminates light pollution for me, and I can capture down to Mag 13 in untracked stacked 50 frame composites....even with obnoxious streelights and parking lot lights all around me....I laugh at them because the higher focal length effectively eliminates them.