Why play 7... Nxd5 instead of 7... exd5 in the QGD? I'm currently following the rapid game Caruana-Nakamura in the London Chess Classic, and I have a basic question about the opening moves.

Why play 7... Nxd5 instead of 7... exd5? Is the idea to trade pieces since black has less space and that would free up his position? Do black's two knights kind of step on each other's toes, making a trade of one knight desirable?

Here's the game: https://www.chessbomb.com/arena/2018-london-chess-classic/04-Caruana_Fabiano-Nakamura_Hikaru.
Both moves are played with exd5 being the more popular choice.  After 8.Nxd5 exd5 is twice as popular as Qxd5 as played by Nakamura.  It is likely Nakamura is trying some ideas in these sidelines in the hopes to catch his opponent unawares.