Be/lie at the bottom of something
Definition 1 of 2
Pro Tip 1/3Uncover Hidden Causes
Used to reveal deep-seated reasons behind actions or situations. 
He believed that selfishness lay at the bottom of their refusal to help.
Pro Tip 2/3Figurative Use
The phrase is often used metaphorically and not to be taken literally. 
Curiosity is what lay at the bottom of her constant questioning.
Pro Tip 3/3Emotional Connotation
May imply a negative or serious underlying issue. 
Jealousy lay at the bottom of his outburst.