Montero, whom the M's acquired from the Yankees as part of the Michael Pineda deal, was ranked by Baseball America as the sixth-best overall prospect coming into 2012. This season, however, Montero has authored a grim batting line of .208/.264/.327 in addition to playing spotty defense in his 225 1/3 innings behind the plate.
Last season, Montero showed promise by putting up a passable 95 OPS+ and making strides in the second half. In 2013, though, he's obviously failed to maintain those gains. Montero's still just 23 years old, so there's plenty of time to right himself. Consider this a bold step in that direction.
Also worth noting is that Mike Zunino, the Mariners' top prospect, is the regular catcher at Tacoma right now, and he already has a backup in place. As Divish points out, that would suggest Montero is going to be shifted to a less demanding defensive position. It seems likely he's destined to become a full-time DH, which, of course, raises the bar for him offensively.
In order to replace Montero on the active roster, the Mariners have recalled catcher Jesus Sucre.