From docs here:
http://docs.mongodb.org/manual/reference/command/replSetStepDown/#replSetStepDown

New in version 2.0: If there is no secondary, within 10 seconds of the primary, replSetStepDown will not succeed to prevent long running elections.

If replSetStepDown is called in a replica set with no valid new primaries (e.g. secondaries in RECOVERING or hidden nodes), and a hidden node is within 10 seconds of the primary, the primary steps down but a new primary cannot be elected.

This results in a replica set consisting of all secondaries. replSetStepDown should throw an error if there is no new primary to be elected, even if another unelectable node is within 10 seconds.