It takes a lot to frighten Steven Rogers, so the moment Thor notices that borderline panicked look in the other man's eyes, he has Thor's full and undivided attention, any trace of humor fleeing his face. What has been going on back on Earth during his absence? Banner hadn't known, for obvious reasons, and Heimdall's been far too busy recently to report on anything outside of Ragnarok itself.

Even the hasty assurances do little to put him at ease, and justifiably so, once he hears the rest. That the politics would be complicated comes as no surprise - as king, Thor pretty much expects that a lot of his duties now are going to revolve around committees and diplomatic meetings and so forth. But the reality of it is nothing like he was expecting, and he listens with mild disbelief as Rogers explains the Accords to him.

"...this is what scattered the Avengers?" It doesn't seem like a far leap in logic, given what he's heard so far, and what he knows of his teammates.