How can somebody fracture my scaphoid?

Many ways: There are many ways to fracture the scaphoid, but the most common mechanism is a fall onto the outstretched hand.

Wrist fracture.: Wrist fractures are most commonly sustained when falling onto an outstretched hand to break the fall.

It's Difficult: You can fracture your scaphoid by falling on an outstretched hand. Somebody can fracture your scaphoid by causing very forceful dorsiflexion of your wrist. Dorsiflexion is bending your wrist so the the top of your hand move towards your elbow. Possibly protecting yourself from a blow could cause this. I don't believe somebody could do it to you by twisting your wrist.

Trauma: Scaphoid fractures take a significant trauma to develop - a hard fall, motor vehicle accident, skiing injury.