When a nuke fired from a fast-moving ship is shot down, the visual shockwave appears to keep moving with the speed of the ship the nuke was fired from. But the actual explosion damage seems to expand relative to a fixed position on the grid.
Would it make sense for nukes to not explode at all when shot down? Right now there are some cases where there's virtually no point to shooting down a nuke, since a ship can move on top of the expanding explosion and take damage anyways. This is also how modern-day nukes (and most military explosives) work - the uranium/plutonium/whatever is too stable to explode unless the bomb is intact.