[Lord Halsar]

Well the Witchking may have been mighty and merciless, but it seems that he can be rather easily goaded into battle, as we certainly saw by his encounter with Eowyn.
No doubt Earnur would have occasionally mocked him over his loss of Angmar. So it was likely a matter of pride for our tall and gruesome freind.
And even accepting the Witchking's challenge (however delayed it may have been) seems rather arrogant and possibly insulting to the Witchking. I am not meaning to draw any conclusions by my next statement, so please don't make it sound like I am. But what better fate for one that had drawn the Witchking's rage as much as Earnur, than to become the very thing he had fought so bravely against. This is only speculation however, and comes simply from my understanding (If it can be called such) of the Witchking's hatred of Earnur and the Gondorians.
But as I have said before, we really don't know.