Following scenario reproduces this:
7 node cluster.
All are running for election for a topic.
node 1 is at the top of candidates list.
node 1 is restarted (killed)
node 2 is automatically elected as leader.
node 1 after restart does not re-enter the race. But continues to reamain the top candidate.
When the current leader is killed (or withdraws) no leader can be elected.