That's only half the question because this leaves the Dolphins with 52 on the active roster. I expect to see them add somebody to the active roster shortly.
While it may be true that they will add someone shortly, I don't think it was the motivating reason for the release. It is more likely that they don't want to guarentee his salary and pretty much have to let him go now.
I don't believe they are pressured to sign someone right away. It may be better to wait after week one on a veteran so that their salary isn't guarenteed.