Over at the Phinsider, Kevin Nogle speculates that the Dolphins weren't happy with his progress recovering from a fluke knee injury he suffered before their first preseason game. As a result, they opted to keep Matt Moore as their primary backup behind Ryan Tannehill:
The Dolphins have since named rookie Ryan Tannehill the starting quarterback, but kept Garrard on the 53-man roster, along with Matt Moore and Pat Devlin. Speculation said the Dolphins were looking to trade Moore, with Garrard serving as Tannehill's primary backup. However, it appears the team was not happy with the progress Garrard was making after his surgery, and are ready to move on from the former Jacksonville Jaguars quarterback.
You never can say never in the QB-desperate NFL, but this may be the end of the line for Garrard, a nine-year NFL veteran who took the snaps under center for the Jacksonville Jaguars for most of the last decade.