If a entity goes to sleep while having the slow falling effect/receives the Slow Falling effect while sleeping, the entity will start shaking. This shaking animation ends when the entity wakes up from the bed or when the Slow Falling effect lasts.

This issue occurs when:

-a player or a villager goes to sleep

-an entity is forced to sleep in a bed using a command like: /execute as @e[type=!player] run data merge entity @s {SleepingX:x,SleepingY:y,SleepingZ:z} (where x,y,z are the coordinates of a bed(head or foot part))