'A shrub' or 'plant,' as in Genesis 21:15, denotes perception, but so little that it is hardly anything, in same way as 'trees,' but to a lesser degree. So 'to be cast under one of the shrubs,' signifies being destroyed regarding truth and good, to the point of desperation.

(References: Arcana Coelestia 2682 [1-3])

It's easy to see the connection between judging and truth. In a court of law, the whole purpose of the judge is to find the truth. Any time anyone judges anything, we hope it is on the basis of the truth. It's no wonder, then, that judging and judgment in the Bible represent truth being put to work. Depending on the circumstances judging can be teaching truth, determining someone's character from truth or setting order to things according to truth. There is a twist, though -- when the Lord is referred to as a judge, it actually signifies Divine Good, rather than Divine Truth. This is because the Lord judges us ultimately from love, and his truth flows from that love.