MongoDB failover reasons
Here is what you can do to start

Run the following on the old PRIMARY

var dt = new Date(db.serverStatus().localTime - db.serverStatus().uptime*1000).toString() ; dt


This will print the exact time mongod was started.

PRIMARY failover was triggered 2015-03-03T12:18:41.540-0500. 


If dt is close to 2015-03-03T12:18:41.540-0500

mongod crashed or was restarted
Check the mongod Log File on the old PRIMARY for that same timeframe

If dt is not close to 2015-03-03T12:18:41.540-0500

mongod is still running fine
The old PRIMARY simply became unreachable
Check network logs along port 27017