shout, chew, eat
咋 centers on forceful oral actions: shouting loudly, chewing vigorously, or eating in a rough, noisy manner. The meanings share a sense of the mouth being used with intensity.
咋 combines 口 (mouth) with 乍, which likely contributes the sound and may suggest a sudden or forceful action. The exact historical development is uncertain.
A mouth (口) making a sudden, forceful sound (乍): imagine someone shouting 'Zha!' while chewing loudly or eating with gusto.
For サク, picture a sack of food being chewed noisily: sack -> サク, and the mouth (口) is working hard on it.