What are some reasons that a switchport interface would be in a Down/Down state?

A) Bad cabling
B) Remote interface is configured with the shutdown command
C) Local interface has been shut down by port-security
D) Duplex mismatch

A, B
Explanation: A) An interface would be in a Down/Down state for several different reasons, including bad cabling, disconnected ports, remote side shutdown or disabled (other vendors), speed mismatch, and remote side disabled by port-security. If the local switchport interface was shut down by port-security, its status would be Down/Down (err-disabled), and if there was a duplex mismatch, the interface would not go down (it might not work, either).
B) An interface would be in a Down/Down state for several different reasons, including bad cabling, disconnected ports, remote side shutdown or disabled (other vendors), speed mismatch, and remote side disabled by port-security. If the local switchport interface was shut down by port-security, its status would be Down/Down (err-disabled), and if there was a duplex mismatch, the interface would not go down (it might not work, either).