It has less insulation between the lowest point of the water and the cold air above it. All else being equal ice freezes to uniform thickness. The shallowest parts of a pond or river are the most likely to down to the ground. There’s several other factors, but the biggest one mostly comes down to how deep the water is and how fast its flowing if at all.
Shallow water freezes first. Ice acts as an insulator so deep water will get cold but not freeze.
This is likely caused by vegetation at the bottom rotting and the gas rising up till it collects in on area, making the ice thinner and thus higher.
Not following why shallow water freezes first.
It has less insulation between the lowest point of the water and the cold air above it. All else being equal ice freezes to uniform thickness. The shallowest parts of a pond or river are the most likely to down to the ground. There’s several other factors, but the biggest one mostly comes down to how deep the water is and how fast its flowing if at all.