The cast from int[2][2] to int[][] is invalid - you cannot slice across multiple dimensions.

import std.stdio;

void main(string[] args)
{
    int[2][2] e = [[1,2],[3,4]];
    int[][] c = cast(int[][]) e;

    writeln(c.length);    // 1
    writeln(c[0].length); // 8589934593
    
    int[2][] valid = e; // no cast needed
    writeln(valid);     // [[1, 2], [3, 4]]
}

The length of a static array is only known at compile time while the length of a dynamic array is a runtime value. Hence c's length is set to some garbage from the stack.