Earlier quoted context omitted.
Perhaps they only find the increase shocking on the left. There's a bunch of ways to measure political opinions. The Authoritarian-liberal one being one of many. The economic-left and the economic-right are becoming more separated from the social-left and the social-right. Tribalism also causes people to take on the positions of their 'tribe' which may be distinct to what their own personalities might normally gravit…
Isn't that just what happens when you keep pushing the Overton Window to the right? What would have been 'centrists' have to become more authoritarian to stand ground or else they let their position get absorbed by the stronger leaning side. When one side refuses to compromise even slightly, you have two options: give in or dig in.
It's rather more likely that the increase of left-authoritarianism is due to increasing resistance to moving the Overton Window, both the left-most edge and the right-most edge, leftwards. As resistance increases more forceful techniques are necessary.