A transaction can get dropped and replaced when a newly created transaction with the same `FROM` account nonce is accepted and confirmed by the network. And because it has the same account nonce as the previous transaction it replaces the previous txhash.

Common reasons a replacement transaction is broadcasted :

1) The earlier transaction had a low gas price which would take a very long time to confirm, so a second transaction with a higher gas price was created to replace it

2) The Ethereum Node that the wallet/service was connected to was not fully synched and an incorrect nonce was used

3) User initiated to replace or cancel a pending transaction

For additional assistance, please contact the sender/transaction creator directly for more information. 

a) Sample transaction that was dropped

b) Sample replacement transaction (notice how both transactions was sent from the same account with the same nonce value of 216474)