Where is the retribution buff coming from?

I just played the brawl and got Tyrion. I played him, and he was 7/6 for no apparent reason.

He had a buff called retribution coming from this, but I have no idea what invoked it. What gave him that buff?

The Retribution buff comes from Bolvar Fordragon.

In your comment you affirmed that the Tyrion being played was actually a Shifter Zerus. The retribution effect must have been triggered on a previous turn when the Shifter was transformed into Bolvar Fordragon and you played a minion from your hand.

This behavior is documented in Shifter Zerus's gamepedia entry:

Buffs from cards like The Mistcaller, Grimestreet Outfitter, Smuggler's Run, and any other card that provides attached enchantments to Zerus will not be removed by Zerus' transformation effect. This also includes cost reduction from Emperor Thaurissan and Unstable Portal. Additionally, if he is temporarily C'Thun or Bolvar Fordragon, he can gain relevant buffs and keep them even after transforming further