Well what I can find online suggests that the DNC is taking the threat of 3rd party candidates more seriously than they have before, which does include legal challenges for ballot access wherever they can make them.
But I thought you were talking about a nefarious campaign to prevent Stein from running specifically, rather than just a heightened effort against potential spoilers in as many states as possible. Polling suggests it's going to be tight, and losing ~2% to RFK/Stein/whoever might tip a battleground state Trump's way.
I never paid much attention to US politics before 2016, but none of it seems out of the ordinary for the last few political cycles at least.