The whole situation began as Chilton was reportedly suffering "contractual issues" with the team. Rossi signed on with Marussia in July as a reserve driver for the rest of the season. With Chilton out, the American was the natural replacement. Apparently, the problem wasn't that serious, because Marussia put out a statement on Friday saying "following a change in circumstances," Chilton got his job back, according to Reuters.
The switch happened so quickly that Rossi drove the car in the morning practice session for the grand prix, but Chilton replaced him later in the day. The team isn't talking specifics about what precipitated the move. "I don't think it would be appropriate for me to comment on the details of the contractual issues because that is confidential between parties involved," said Marussia team president Graeme Lowdon to Reuters about today's announcement.