We never got an explanation during the game. The broadcast crew didn’t know why either.
But there’s this from after the game, though it doesn’t say what he did:
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from Rams DE Eugene Sims ejected from game for unsportsmanlike conduct
link: http://www.todayspigskin.com/pigskin-news/rams-de-eugene-sims-ejected-game-unsportsmanlike-conduct/
The penalty happened after the Buccaneers scored a touchdown to take a 13-10 lead. On the extra point attempt, Sims got into an argument and pushed it past the point of being acceptable for the ref, causing the ejection.