It makes sense in one respect. Putting Mellette, a rookie seventh-round pick out of Elon, on IR ends his season, but it's one way to keep a developmental player. If you try to put him on the practice squad, you risk him getting picked up by another team.
Still, there was no indication that this move was coming or that Mellette was even injured. It was announced that Mellette has a knee injury. Mellette wasn't on the Ravens' Week 1 injury report (which means he had full practices) and he didn't play in the season opener. The Ravens did practice twice since Thursday's game, but coach John Harbaugh sounded optimistic about Mellette playing at some point this season after Monday's practice.
"He’s playing well," Harbaugh said of Mellette. "There’s nothing that he’s doing that wouldn’t make him a possibility to be active."