C uses row-major ordering to store multidimensional arrays, as documented in § 6.5.2.1 Array subscripting, paragraph 3 of the C Standard:
Successive subscript operators designate an element of a multidimensional array object. If E is an n-dimensional array (n >= 2) with dimensions i x j x . . . x k, then E (used as other than an lvalue) is converted to a pointer to an (n - 1)-dimensional array with dimensions j x . . . x k. If the unary * operator is applied to this pointer explicitly, or implicitly as a result of subscripting, the result is the referenced (n - 1)-dimensional array, which itself is converted into a pointer if used as other than an lvalue. It follows from this that arrays are stored in row-major order (last subscript varies fastest).
Emphasis mine.
Here's an image from Wikipedia that demonstrates this storage technique compared to the other method for storing multidimensional arrays, column-major ordering:
The first function, sum1, accesses data consecutively per how the 2D array is actually represented in memory, so the data from the array is already in the cache. sum2 requires fetching of another row on each iteration, which is less likely to be in the cache.
There are some other languages that use column-major ordering for multidimensional arrays; among them are R, FORTRAN and MATLAB. If you wrote equivalent code in these languages you would observe faster output with sum2.
