Probably just a gameplay thing, although it is kinda weird for a weapon as legendary as Gorehowl is an entry level raid drop. It makes sense that someone from the Legion would have it, guess it just wound up in his hands and subsequently mine as well
We dont explicitly know what happened, but Thrall had Gorehowl. Grom died alongside where Thrall was. It seems pretty simple that he'd just take the axe since it was pretty iconic. The weapon that slew Mannoroth.
Sky Admiral Rogers for High Queen. Remember Theramore!Men, you've trained for this. You're among the elite. You are Skyfire men! This is the ship that took down Deathwing. You think some raggedy little Horde outpost stands a chance against the pride of the Alliance fleet? NO! Those green dirtbags down there plagued your homes in Southshore, laid siege to your children in Redridge, and massacred every man, woman and child in Theramore. It. Is. PAYBACK TIME!
It could be possible that demons did indeed snatch it away after Mannoroth's defeat, and then Lore-wise, one of the horde adventuring parties killed Malchezzar, and took the axe to Thrall?
Well, Karazhan is one big jumble of different times and places intersecting, which is the very reason the legion is so interested in it, so maybe some effect of that put Gorehowl into Malchezaar's hands.
Or the "Ockham's razor"-option: Blizz wanted to throw the "Warcraft III"-horde fanboys a bone by making that supposedly emblematic weapon available to players to use (Personally think it's overrated, but i play alliance :P )