NFL Free Agency isn't exactly a game of Russian Roulette, but it always involves a level of uncomfortable risk. Players typically reach free agency for one of two reasons: They're not that good anymore, or they have lengthy injury histories.
There are rare instances in which All-Pro caliber players reach NFL Free Agency due to the fact that the team they are leaving simply doesn't have any slice of the salary cap pie to give to them anymore, but for the most part, you can bank on guys hitting free agency for one of the two aforementioned reasons.