My guess is this:
Graham $7 mil ($10 mil with Seattle, deemed too much. Another year older; strictly a pass receiving threat).
Bowman $3 mil (earned less than $2 mil with Oakland)
Ngata $3 mil (coming off injury; 34 years old; 2 down player)
So there is $13 mil in cap space for those 3. Cut Barron and over half of that is paid for. Cut Austin and that leaves a difference of only $3 mil (if my numbers are in the ballpark for the FAs).
These are the kinds of players you pepper in with the younger guys to give leadership and help in specific areas.