That's... well, it's certainly an answer. Or rather it's some words masquerading as one and Steve's gut reaction is to immediately call bullshit, except--

"Bucky, did someone threaten you? Did they threaten your sister?" That's it, right? The explanation for what could get a man to go from hot to cold like a switch was flipped. What's Bucky's top priority? What could understandably trump anything else?

The problem is that Steve can't sit back and let Bucky face trouble like that by himself, so if he's facing it, Steve is facing it with him. There's a whole narrative building in his head, because Steve can't quite outgrow those principals no matter how much the world slaps him in the mouth for having them.