So says mlb.com's Todd Zolecki:
To call this surprising would be an understatement: after being tendered a guaranteed contract over the winter, Frandsen was considered a strong favorite for a bench job all through the spring, and the recent injuries to infielders Freddy Galvis and Darin Ruf seemed to cement his place on the Opening Day roster. In 34 at-bats this spring, Frandsen had seven hits (all singles) for a .206 average. It's likely, though, that the injuries contributed to this move, as the team will need to clear at least one spot for players not currently on the 40-man roster.
With just eight days until the season begins, this leaves Ronny Cedeno, Reid Brignac and Cesar Hernandez in the backup infield mix. Outfielders John Mayberry, Jr., Tony Gwynn, Jr.. and Bobby Abreu remain in competition for bench jobs as well.
Frandsen is due $900,000 this season. If he rejects the triple-A assignment, he becomes a free agent and, per Matt Gelb (and contra what I initially wrote), the guarantee is voided:
Kevin Frandsen has 72 hours to accept an outright assignment to AAA or elect free agency and forfeit $900,000 salary.— Matt Gelb (@magelb) March 23, 2014