If an entity (player, mob or other entities like the armor stand) stands on a bed or on a slime block and gets the Slow Falling effect (via potion, command or area effect cloud) it will start shaking. This shaking animation will end when the Slow Falling effect lasts or the entity walks off the block.

However, this issue will not occur when a player, standing on a bed/slime block, gets the Slow Falling effect while pressing Shift.