As in things of our physical well being, so also there is a cause and effect relationship for the well being of our souls and for eternal matters. The proverb quoted tells us that the curse does not come "causeless". Other people may call curses upon us, but if there is not an underlying cause before God they are powerless. Human hatred expresses itself by calling upon God to damn others or by wishing them to hell, but God alone has the power of blessing or cursing.
The proverb seems to be comparing the causeless curses to birds flying about overhead - but never landing upon us as there is no reason to. The swallows dipping and swooping may seem to threaten, but they are aiming for other prey. There is only one sufficient cause that will bring without fail the curse of God, and that is sin. That is a basic cause-and-effect law of the Almighty. God is holy and brings His curse to rest upon the head of the one who breaks His laws. The sobering fact is that we all have sinned.
The good news is that the curse can be removed. God is not only holy, but also merciful, gracious, and loving. He has planned and provided a way that both the underlying sin and the consequent curse can be removed. "Christ hath redeemed us from the curse of the law, being made a curse for us: for it is written, Cursed is every one that hangeth on a tree" (Galatians 3:13). When God's Son died on the Cross, He was taking our curse upon Himself. This is a finished work and the resurrected Lord today offers to sinners today forgiveness of sin. Through turning from sin to Him in simple faith we receive instead of a curse, the promise of eternal life.