Leaf springs that snap at the shackle leaf normally indicates an overload condition. The upper springs have absorbed all the weight that they can so now all the weigh is supported by the lone shackle leaf. The spring reaches it's deformation point, and it snaps.
'07 K3500 Silverado LT Crew Duramax (LBZ)
2016 Salem 27RKSS
1984 CHEV SCOTTSDALE K20 2GCGK24J0E1XXXXXX (Chevrolet Legends-Class of 2019)
"...exhaust fluid? We don't need no stinkin' exhaust fluid"