How are the strikers able to "slip surprises " into the workers' harvesters? Are these good or bad "surprises?

Strikers slipped things into the harvest as the workers crossed the picket line. The picket lines had to be crossed in order to reach the shed. Besides throwing stones and yelling taunts at the workers, the strikers also employ some pretty despicable tricks to frighten them. The hide things in the crates... living things like snakes and rats, as well as non-living things like razor blades.