Spiral downwards
Definition 1 of 2
Pro Tip 1/3Metaphorical Use
Often used to describe worsening situations metaphorically, not literal falling. 
Their business partnership began to spiral downwards after the disagreement.
Pro Tip 2/3Emotional Weight
Can imply emotional consequences, like disappointment or hopelessness. 
Her spirits spiraled downwards when she heard the sad news.
Pro Tip 3/3Visual Imagery
Suggests a mental image of something falling or failing rapidly. 
As rumors spread, the celebrity's reputation began to spiral downwards.