Wasn't it his/her three formation abilities that aren't the attack priority one?

The lack of distinction kind of is the entire joke with that character. Just like Rocket and Groot, it's like Broot has a three word vocabulary that only Robbie understands. (Then again, they could have played with the spelling to make it look like different words were being drawn out as if they're being said with different tones. "I aaam Broot", "I am Broooot", and other variations.)

Thinking it through, it stands to reason that a weekend buff effect wouldn't alter an attack priority ability (because that would do almost nothing) or a level 200 ability (because no weekend buff has ever touched the "x4 every 25 levels" scaling) so that leaves:
(Passive) Increases the Base DPS of Broot by 100%
(Passive) Increases the DPS of all Crusaders by 10%
(Formation) Increases the DPS of all Crusaders by 25% when Broot is in the front column
(Formation) Increases the Base DPS of Robbie Raccoon by 100% when Robbie Raccoon is behind Broot
(Formation) Increases the Base DPS of Robbie Raccoon by 100% when Robbie Raccoon is adjacent to Broot

Actually... Have any of the weekend buffs ever affected an existing passive? It seems like it's always either a buff to a formation ability or a separate passive effect exclusive to the weekend buff, but I could be wrong.

Anyway, I recall noticing during that weekend that Robbie got buffed way up indirectly through the buffs to Broot. So that at least clues you in that the two abilities that mention Robbie were probably two of the three that got buffed.