I've noticed this as well. I apologize for my ignorance, but is there any reason for it? Did Avengers Arena really cause that jump?
All I know is that I bought one out of nostalgia. How many others are doing the same is anyone's guess. But the whole kings of kings story line might of given him just a bit more spotlight for those unaware of the character. And after the kings of kings he's more of a universe type entity/character and since the Guardians of the Galaxy movie announcement, Marvel space characters have become hot.
All theories of course.
I've been watching/selling/buying this book for some time now. The current upswing, according to the recent buyers I've talked to seem to be hinged on pure nostalgia. In 1991 this was a hot character and people back in the day are remembering again and seeking #1s out. I don't think some of these buyers are even vaguely aware of the more recent Darkhawk appearances but yet there is growing pent up demand for HG #1s. Also its 22 years later and the 25 year rule is coming into fruition here (why is this in the modern thread again?) .
btw, If anyone has the DH in my signature line send me a pm.