The Packers receiver was just trying to help his coach.
Coaches are not allowed to challenge plays that are automatically reviewed by NFL officials, like fumbles. Throwing a challenge flag results in a 15-yard unsportsmanlike conduct and, if the flag is thrown before the booth buzzes down, the challenge is nullified.
It didn't work. The play was still challenged, because the booth buzzed before the flag was thrown, but McCarthy was penalized 15 yards as a result.