Mongodump on Replica Set
Its not a good idea to run the backup against the Primary. The extra load you adding on the Primary might cause heartbeat failures and trigger an election. Now in your case, overloading the primary might caused an election, you can see that info on your logs, but the other secondary didn't become new primary which expected to. I see some cases why didn't. 1) Replication lag was big 2) you run the replica set on the same server or storage so all nodes get affected but here logs are your best friend on what really happened.