The wave could have been traveling way faster than light in some weird subspace realm (one that formed a nice, planar channel of expansion for it), but "leaking" to normalspace all the time, too. The slower-than-lighspeed wavefronts would then have been interference patterns of this leakage. We observe something like this with radio waves, which form standing or slowly creeping waves out of lightspeed wavefronts.
Alternately, the wave first blew out to deep space at high FTL, then lost enough of its energy to drop back to sublight before encountering the
Excelsior. Remember the doubletalk in TNG Tech Manual about how unboosted subspace signals degrade into lightspeed ones after a certain distance (about 22 ly in that case)? The same could have happened to the Praxis wave.
As regards the resources left on Praxis, we don't really know what
was on that moon. It was a "key energy production facility", but most nuclear plants aren't built on top of uranium mines; and few coal or oil plants sit next to their fuel sources, either. Perhaps the Klingons simply had an incredibly big powerplant on the dark side of their moon, using imported fuels in a fashion that would have caused lots of pollution or other risk if done planetside.
Praxis succumbed to "overmining". Doesn't necessarily mean that fuel was being mined from the depths of the moon. Could be that the engineers dug one tunnel too many when creating undersurface fuel depots, and collapsed something. Or then the Security Division got too zealous in applying anti-intruder mines on the corridors.
Timo Saloniemi