I see lots U.S. wildfires on Zoom Earth, and lots news about them. Is this amount of wildfires normal? Or is this average? If over average, why?
For now it seems to be average, at least compared to the last decade. The National Interagency Fire Center has a page with a year-to-date statistics table. You can see that so far, the values for 2020 are pretty similar (even slightly below) to the 10-year average, both in terms of number of fires and burned acres.
2020 | Fires: 41,599 | Acres: 5,288,247 2010-2019 | Fires: 44,177 | Acres: 5,761,034
Now if you go further back in time, there seems to be an upward trend. I did not compute the means, but just by looking at the numbers you can guess that there were less fires in the 80's and 90's, and about the same amount as today from the 2000's.
For now the destruction of the US Pacific wild fires is well above average. As of 11 September 2020,
the fires have obliterated California’s record for the number of acres burned in a year. Six of the top 20 largest fires in state history have occurred in 2020, according to Cal Fire.
The reasons for the fires are,
Lightning initially triggered many of the fires, but it was unusual and extreme meteorological conditions that turned some of them into the worst conflagrations in the region in decades. Record-breaking air temperatures, periods of unusually dry air, and blasts of fierce winds—on top of serious drought in some areas—led fires to ravage forests and loft vast plumes of smoke to rarely seen heights.
... a perfect storm of meteorological factors come together that encouraged extreme burning ...
... on top of shifting climate patterns—a long term drying and warming of both the air and vegetation—that is contributing to the growing trend we are seeing toward larger, higher-intensity fires in the U.S. West.
The buildup of fuels may be another relevant factor. Human efforts to extinguish most fires over the past 120 years has led to an increase in old, overgrown forests in the West that burn intensely when they catch fire.