You bow to your parent, to the guru, master and whoever you respect. What about the nature?
The nature has provide us everything we need to survive. The earth, the air, the sun, the water, and when was the last time you feel absolutely grateful to the nature that has given us so much but never ask for any return?
In yoga asana, forward bending pose bring calmness, humbleness, and the sense of surrendering to the practitioner.
Be humble, be sincere. Bow down to the nature and surrender. You are not a human seeking for spiritual journey; You are the spiritual being having a human experience.