These are obtained by counting up in base 16 and then counting down in base 16.
Specifically we break the 4x 32-bit words into 32 4-bit pieces and treat each piece of 4-bits as a hex digit (equally you can think of this as breaking each byte in half). We set these first 16 of these hex digits to be the numbers 0, 1 ,2,... f and the next 16 of them to be f, e ,d , c,..., 0.
Incremental counting is considered a natural enough operation that these are not "magic numbers", but rather a good example of "nothing up my sleeve" numbers.