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.
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.