Stacking refers to the practice of loading your lineup with multiple players from the same game or the same team. It is a widely used strategy, and most major DFS platforms allow it.
The Angels are coming off a split in Houston, and like the Nats they are not projected to be particularly good this year. In Saturday's loss, one of their relievers put up a fun 1 IP, 6 R, 0 ER line thanks to multiple errors and walks, and was rewarded by being shipped to the minors the next day.
When a player sometimes plays and sometimes doesn't, there's a lack of confidence in key moments because you don't feel the pitch, you've just come on, or you haven't played in the last two or three games. Sometimes there's a moment of hesitation.