Why were trenches formed in a zigzag pattern in world war 1?

They were in a zigzag pattern to prevent enemy gunfire from just killing everyone by firing in a straight line in case an enemy ever gets into the trench.

The answer lies in knowing what would happen if trenches were formed in a straight line. Imagine it! If trenches were in a straight line and a portion of the trench was overtaken by the enemy, it would make it so much easier for the enemy to quickly open fire down the length of the trench to take out any soldiers harbouring there and thus quickly take control of the trench before anyone could muster a counter attack.