A single RDF link in the RDF group is now operational after single link down event, this is occuring randomly. Any advise what could be the reason ?

I had similar alerts in Unisphere in the past.  The culprit was our communications team, which handles our ISL circuits (which is how our SRDF link connects).

Apparently an RDF link will move to the 'link down' state if communication interrupts exceed 10 seconds.

If you open a case with EMC, they can tell you when the interruptions are occurring, but you'll probably have to monitor your ISL or communications provider in order to determine the why of it.

Please check with your Network team to investigate on this link drop