It's 1000% density/air quality. The only time my truck acts funny is when it's hot and humid, the higher the humidity - the worse it is. Warm and dry is fine, Cool (sub 60s) and humid is fine, cold (rare for us) is like a whole new beast, but the factory tuning seems to have some BS limit on how crap the air can be (or it just goes out of bounds and does dumb things). I get the P0402 code only when loaded and high temps/humidity (yea I know I have the banks elbow, boost tubes, and the S&B intake), but it only does that crap when its hot and swampy. I've also noticed it's worse if I start the day close to sea level and then end the day going up into the Texas hill country. That elevation change + hot + humid = annoying things (The Service Exhaust Alerts and then P0402 CEL killing my remote start).
What I've noticed when the seasons change is this with my truck in South Central Texas -
"Winter" - Truck is real fun to drive, turbo spools good - TX "winter" fuel MPG sucks, but the truck runs GREAT. DEF Consumption is like nothing and the DPF is almost always clean unless I have too much fun on back roads without driving it down the hwy after. No code, no annoyances, I want this truck all year long.
Spring - Truck can run from ok to great, but the 95%+ humidity days = the time of the DPF loading and needing to "run" it out with towing if possible. Starts getting DEF thirsty
Summer - Truck throws the damned P0402 codes and GUZZLES DEF, but the DPF is almost always clean. It's the most "pigish" in summer, she ain't the fun girl she is in the cooler temps at all.
Fall - Same as spring, but the same humidity problems. It's the time of DPF loading again and it gets better along with DEF consumption dropping as it cools down and drys out.