plow soil, place name
垡 centers on turning over soil with a plow; the place name sense is a specific application of that agricultural idea.
垡 combines 伐 (likely phonetic) with 土 (earth), suggesting an action related to soil. The exact historical development is uncertain.
伐 suggests cutting or striking, and 土 is earth. Picture a blade cutting into the soil to plow it.
For ハツ, imagine a farmer plowing soil while wearing a hat: hat -> ハツ, and the hat shades the work of turning the earth.