Any new regulation the EPA introduces results in litigation. Some of the previously introduced PFAS regulations weren't done in accordance with how the Safe Drinking Water Act says they should be (regulations were introduced without the necessary public consultation), so they're applying to partially vacate the previous ruling. Notably, they're _not_ applying to vacate the regulation of PFAS chemicals where they say the process was followed correctly.

So, the legal reasoning might be to cut their losses litigating to defend rulings they think they'll lose due to the administrative error. I also suspect that being seen to roll back some regulations likely gives Lee Zeldin (the EPA admin) some political room to maneuver. He's historically be associated with anti-PFAS efforts (in Congress he represented a district with contamination problems and he voted for anti-PFAS legislation), but he's also part of an administration with a strong anti-regulation agenda, so he needs to walk a fine line.