moxa, sagebrush, wormwood, mugwort
艾 centers on aromatic plants used for medicine and burning: mugwort, sagebrush, wormwood, and the moxa made from them.
艾 combines 艹 (grass/plant) with 乂, which likely contributed the sound and may suggest cutting or harvesting the plant. The exact historical development of 乂 is uncertain.
The grass top 艹 sits above a crossed shape 乂, like bundles of dried mugwort tied together for moxibustion.
For カイ, imagine a kite (カイト) made of dried mugwort leaves flying above the crossed bundles: kite -> カイ.