[DX] Can anyone explain this?

[Robert Brown]

Bill, 

Looking at the path from XZ to W9, gray line considerations hardly apply
at the time you mentioned, 1800 UTC.

As for the opening on 15 CW at that time, that depends on critical
frequencies all along the path supporting propagation on 21 MHz.  MUF
considerations do not apply as they only involve critical frequencies at
the two end-points of the path.

Breaking the path down into hops, it is clear that the weakest point, as
far as critical frequencies are concerned, is in the north polar region,
around 84N, 131E.  So your opening was created there, probably from a
fluctuation in the F-region ionization.

Most people are unable to break paths down into hops and see what is
really going on.  But intermediate hops are important, especially on paths
that go across high latitudes.

73,

Bob, NM7M