Living as I do in a suburb of the San Francisco Bay Area, I believe it is certainly true that wildfires have been larger and more prevalent in the past few years than before.
Inland Northern California experiences two weather seasons: a cool and wet winter and a hot, dry summer. While we in the Bay Area might experience modest grass fires and the like in August, the usual fire season is in the fall transition between these two seasons, when warm, dry winds can occasionally really get going but before the rains dampen everything. Lightning is, however, a common summer occurrence in the forests of the Cascades and Sierra, and so these areas do experience frequent, albeit generally smaller, mid-summer fires.
This past months' fires were caused by a rare major regional lightning storm, the result of moisture from a Baja hurricane remnant somehow making it this far north and interacting with our normal summer airmass. For those of you who don't live here, know that severe mesoscale thunderstorms are almost unheard of in the Bay Area. Unfortunately, this is also the time of year when winds are light and an inversion layer is a daily occurrence, meaning the smoke stays trapped for days, or in this case even weeks. Smoke from fires up to a couple hundred miles away can blow or drift from this area to that, with obvious repercussions for DSO observing. In our current instance, the smoke has been so thick I can smell it, and I've been keeping my telescopes inside to avoid exposure to particulates.
As others have stated, a confluence of factors has combined to support conditions necessary for enormous fires. To give you a sense of the enormity, for example, the "SCU Fire," whose northern border is about 12 miles away from me, has burned something like 625 sq miles, equivalent to a square about 25 miles on a side. Another almost equally large fire was located about 50 miles north, and there are also several fires of > 50 square miles within, say, a 100 mile radius. First, fires were suppressed for decades in these areas, meaning scrub and dead trees have built up. Second, as one post explains, we experienced a major drought during the 2011-2016 period. This, along with a related bark beetle infestation, killed off a significant fraction of our conifers and, as they say, added fuel to the fire. Third, the lightning storm started many small fires over large geographic areas, causing fire suppression resources to be dispersed and insufficient. Thus, the blazes grew together unabated into monsters.
Now California is heavily urbanized. Fly over the state and you won't believe how much undeveloped land there is, by far the vast majority. Much of it is public land: national forests, BLM, certain large military bases, various regional park entities, etc. The good news is that much of the undergrowth has now been burned out. The bad news is that plenty remains, in particular on land adjacent to large population centers such as the SF peninsula. For example, the "CZU Fire," which burned in a redwood forest south of San Francisco (the larger redwood trees are highly fire-resistant, BTW) is another example of a lightning-caused fire that grew out-of-hand.
We here in NorCal are likely to experience additional large wildfires due to plentiful fuel and a slowly warming and overall slightly drier climate. However, these will mainly occur in the fall, be wind-driven, and without the smoke lingering nearly as long. I hope they won't all happen at once though, as they did these past few weeks.
Happy observing always,