"mostly ATVs spinning little tires that do the digging,"
I also think the bigger vehicles with larger tired (37s everywhere on jks now) vehicles that are common moved rocks and boulders that used to stay in place. Its a sort of Occum's razor with bigger tires needed to clear the more torn up trails, but then it seems the bigger vehicles move more stuff around making it that much more difficult. Actually, maybe its not the size of the vehicle or tire, but rather with modern suspensions etc, people can drive over stuff a lot faster and maybe that is the issue. Driving at crawl speed would help minimize trail impact and damage. In my old jeep, I rarely get out of 1 or 1.5 low - its too rough, bouncy and will literally break something. But in my Taco I am in 2,3,4 low (with higher gears even), and have tendency to cruise over stuff. Additionally, with extremely capable off dealer lot vehicles - there are a lot more people driving these trails and the more use, the more things seem to get tougher.