For a good explanation of that scenario see the following link fo detials:
"the vPC secondary peer device brings down all its vPC member ports due to dual active detection."
what happens when peer-keepalive goes down 1st and then peerlink goes down, this situation will cause dual active scenario and so split brain, right? in this case, secondary peer device won't bring down all its vPC member ports, right?
When your keepalive goes down nothing much will happen as your CFS will be communicating between the devices.
Now what happens when Peer-link goes down when you have keepalive already down?
AnsweR: you are right on it that now you will have dual active scenaorio and both the switches will be active and secondary peer will not bring down the vpc member ports because there is no one to communicate between the switches about the peer-link is down hence both will be active.