Writing embedded list to CSV in Python
One quick win for readability would be to rename item to player.  Another improvement would be to put each field on its own line, since the long lines vastly exceed the 79-character limit suggested by PEP 8.

For the 16 scores, you can write…

writer.writerow([
    'YEAR',
    'FILENAME',
    'PLAYER_KEY',
    'PLAYER_NAME',
    'SEASON_TOTAL',
    'CALCULATED_SEASON_TOTAL',
    'MEAN',
    'MEDIAN',
    'STD_DEVIATION',
    'CV'
] + ['WK{0}'.format(i + 1) for i in range(16)])

for player in list_of_players:
    writer.writerow([
        year,
        filename,
        player['player_key'],
        player['player_name'],
        player['season_total'],
        player['calculated_season_total'],
        player['mean'],
        player['median'],
        player['std_deviation'],
        player['coefficient_of_variation']
    ] + player['scores'])