ChatGPT says this:
Yahoo Fantasy doesn’t make the “Matchup of the Week” designation random; it uses a combination of projected competitiveness and league/storyline factors.
Here’s how it generally works in standard Yahoo Fantasy Football leagues:

1. Projected Score Difference
Yahoo looks at the pre-game projected point totals for each matchup. The game with the smallest projected margin between the two teams is typically highlighted as the “Matchup of the Week.”
If there are multiple close matchups, Yahoo may favor ones that involve:
• Teams near the top of the standings (potential battle for 1st place)
• Playoff race implications (e.g., two 5–5 teams fighting for a spot late in the season)
• Rivalries (rematches from playoffs, prior trash-talk matchups, etc.)
Yahoo occasionally adds a bit of flair — for example:
• Rematches of last season’s championship game.
• Two teams with long winning streaks.
• Teams with identical records going head-to-head.