...that I could now answer my own question as well, having just downloaded the comic. According to it, the Trow kill any race that comes into conflict with them, save the "younger races", who have yet to really have a conflict with the trow (they fought with balor, but this was his fight, not theirs).
So, the forest giants could not have fought the trow, or they'd be dead and gone. This is the only explaination I find, though it's weak:
"When asked of the source of their animosity towards the Trow, all were silent. After a time the eldest among them looked down and seemed to weep, 'It was they who poisoned the soul of iron.'"
Sorta like Forest Giants were some sort of elemental protectors. Eh, who knows.
I got a real kick out the the trow with swords though. Mmmm.
--Stinger
