IDK,but it would seem to me that talking about and working on past traumas would be what's causing the nightmares and not the grounding you are doing.
Therapy stirs things up,even for those that don't have parts.My therapist called it 'fallout' from therapy and said nightmares,anxiiety,feeling like crap,etc.is part of the process.He also said if I wasn't struggling afterwards it meant therapy wasn't working and that it usually does get worse before it gets better.
But he also kept track of my struggles and increased/decreased sessions as needed.Sometimes he would have me take breaks from therapy to stabalize too.
Maybe a short break might be needed until things calm down?