MLG ACFT IN CRUISE; FL280; RECEIVED TCASII RA. FLC FOLLOWED RA AND DSNDED 700 FT WHEN CTLR INTERVENED TO GIVE A HDG VECTOR TO CORRECT CONFLICT WITH ACFT AT FL270. ANOTHER ACFT DSNDING TO FL290 PROBABLY CAUSED TCASII FROM PROBABLE HIGH DSCNT RATE. CTLR DIDN'T ALERT FLC TO ANY TFC PREVIOUS TO TCASII.

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XING THE CHS VOR AT FL280; WE WERE IN LEVEL FLT CRUISE .74 MACH. WE FIRST RECEIVED A TA ON OUR TCASII DISPLAY. THE ACFT WAS NOT VISIBLE. THE TCASII SHOWED IT IN A DSCNT. THE TCASII THEN GAVE AN RA FOR US TO DSND AT A RATE OF NO LESS THAN 2000 FPM. THE ACFT WAS STILL NOT IN SIGHT AND A DSCNT WAS INITIATED. THE DSCNT WAS HALTED AT FL273 WHEN WE ESTABLISHED VISUAL CONTACT WITH THE CONFLICTING ACFT. IT WAS LESS THAN 1 MI HORIZ AT FL290. AFTER THE VISUAL CONTACT WAS MADE WE STARTED A CLB BACK TO FL280. ATC AT THIS TIME ISSUED AN AVOIDANCE VECTOR FOR US DUE TO CONFLICTING TFC AT FL270. WE ADVISED ATC OF THE RA; CLBED BACK TO FL280 AND RESUMED OUR FLT PLANNED RTE. CONTRIBUTING FACTORS: TURB AT HIGHER ALTS; SMOOTH AT FL270-290 CAUSED A HIGH CONCENTRATION OF ACFT IN A NARROW VERT AREA. THE ACFT THAT GENERATED; OUR RA PROBABLY DSNDED AT A RAPID RATE TO GET OUT OF THE TURB. OUR TCASII DID NOT KNOW THE ACFT WOULD STOP AT FL290. ATC DID NOT ADVISE US DUE TO HVY FREQ CONGESTION OR OVERSIGHT. THE TCASII; WHILE RESOLVING ONE POTENTIAL COLLISION WITH ITS LOGIC; CREATED AN ADDITIONAL POTENTIAL COLLISION.