Lemon, or just doesn’t like it anymore. My mother only goes 90k mi in a vehicle and she goes and gets something else. She’s  just like a lot of Americans. We get bored with stuff and want something new. Or we try to outdo our neighbors and we just need to have the latest and greatest. 

At 1000 miles I doubt it’s a lemon, likely they didn’t like it, or changed their mind. Are you sure it was traded in? Sometimes dealers have demo or loaner cars that are new. Ask the dealer if it had a previous owner. My wife’s 2014 was “new” when we bought it, but had 4k miles on it. Same with her Safari van in 2001. Got a new car at a lot less money basically 

You never said what the Make/Model for 2020 was in your OP.   

I've seen several reasons over the years. Sometimes, they never consulted their other half and he or she wasn't having anything to do with it. I personally benefited with the next two. Person bought a muscle car and had a difficult time keeping it under the speed limit and got pulled over a few times, so they traded it for something they wouldn't be inclined to go too fast. On another occasion, person bought a brandy new '96 Ranger 4x4 5spd., and had to bring it back because they couldn't afford the insurance + car payment, was like 800 miles on that one. For every one out there, I'm sure there is a some story for each one of them

Someone who is picky or just rich was tired of the car, or just didn't like the way it drove and they probably saw a better car somewhere else. My dad's friend traded in 4 cars in a year before finding the right one for himself. 

Check extremely closely for any wreck/flood damage.