Short story or novella from a few decades ago:
A Russian woman, the last surviving human as far as she can tell, has been so far surviving an extreme nuclear winter by traveling in a heavily insulated vehicle from a joint U.S.-Soviet collaboration project she was participating in at the time that nuclear war broke out. (It had been a project sponsored as some kind of relationship-improvement type project by the two governments.)
(It emerges in a flashback that when the ICBMs fell, she actually was responsible for locking out of a bunker an American scientific collaborator, as she was enraged and bitter with grief from the knowledge that her own family and everyone she knew back home was dead.)
As she travels around the wastes of North America in the vehicle (sometimes imagining the howls of impossible wolves -- in reality, everything else is frozen, dead) she comes across a building that is lit.
She meets not surviving humans, but a crew of vaguely duck-shaped aliens. (She dubs three of them them Huey, Dewey, and Louie.) Communication is difficult (the aliens are highly intelligent but have very different mindsets from humans, and have to play-act out certain scenarios amongst themselves to understand new concepts), but it eventually emerges that they are peaceful aliens who had been sent on expedition to meet and study humanity. (I guess they had recognized that there were radio broadcasts emanating from the Solar system.) She fills them in on what she knows about what happened.
Over weeks that they get to know each other and the aliens learn more about what happened, a few individuals of the alien crew commit suicide, to her alarm.
She ultimately learns that during the aliens' maneuvers to begin deceleration at the midway point of their journey to Earth (the alien vessel or vessels had traveled at some significant speed, like .1c or something), there had been an accidental release of material, I think maybe just hydrogen. I think an alien craft might have been lost, I'm not sure. One or more alien craft from the group were able to successfully decelerate, but the stray particles (continuing on at that peak, relativistic speed relative to our system), had enough velocity that when they months or years later swept through the solar system, enough of those high-energy nucleii impacted Earth's atmosphere that the high altitude showers of secondary particles set off nuclear attack detector in the U.S. and/or U.S.S.R., prompting retaliatory ICBM/sub nuclear launches. (I don't remember if it was determined whether one side launched first or both misinterpreted the initial data; if the alien direction of approach was close to the ecliptic it might have depended on which side of the Earth was facing that direction at that time of day/year; in any case, the result was the same: both sides launched their missiles.) The decelerated alien craft arrived a few years after, finding a frozen world.