Per ESPN's Brooke Pryor, DeCastro said he hasn't received an official explanation from the NFL for why he was fined, but he thinks the league disciplined him for being the "peacemaker" in the altercation.
Per an official list provided by the NFL (via ESPN's Adam Schefter), a total of 33 players were disciplined for their roles in the brawl and both organizations were fined $250,000.
DeCastro's name wasn't included on that list, but he told Pryor his fine was $14,000. Browns defensive end Myles Garrett ($45,623 and an indefinite suspension), Steelers quarterback Mason Rudolph ($50,000) and Steelers offensive tackle Maurkice Pouncey ($35,096 and two-game suspension) were the only players fined more.
Browns defensive tackle Larry Ogunjobi received a one-game suspension for an unnecessary roughness penalty when he shoved a Steelers player to the ground during the brawl.
Garrett's discipline stems from ripping off Rudolph's helmet and hitting him in the head with it late in the fourth quarter of Cleveland's 21-7 win. DeCastro got involved in the skirmish by separating Garrett and Rudolph from each other and holding Garrett down by lying on top of him.