Matrix in "slash" order
Jelly, 6 5 bytes

pSÞỤs


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How it works

pSÞỤs  Main link. Left argument: n. Right argument: k

p      Take the Cartesian product of [1, ..., n] and [1, ..., k], yielding
       [[1, 1], [1, 2], ..., [n, k-1], [n, k]].
 SÞ    Sort the pairs by their sums.
       Note that index sums are constant on antidiagonals.
   Ụ   Grade up, sorting the indices of the sorted array of pairs by their values.
    s  Split the result into chunks of length k.