I think a lot of writers were enamored with Cadillac's performance after the first few games and decided to just annoint him Rookie of the Year right then and there.
Either that or they also looked at the 122 yds and 2 TDs he put up "at" Carolina in week 14, or the 150 and a TD he put up against Atlanta in week 16.
Or, more likely, the voters actually looked at his whole season and judged him worthy. 14 games, 1,178 rushing yards and 6 TDs for a team that made it to the playoffs...for a team that wouldn't have even sniffed the playoffs without Cadillac in the backfield.
...sounds like he was deserving of the RoY award, imo.