The following contains spoilers for Hulk #6, now on sale from Marvel Comics
The Hulk has fought almost every notable Marvel hero at some point or another, with his rage-induced rampages taking him across the entire universe. His next showdown with a fellow major hero might finally have a reason behind it and perfectly explains why Thor is going to be willing to face off with him.
Hulk #6 (by Donny Cates, Ryan Ottley, Cliff Rathburn, Frank Martin, and VC's Cory Petit) featured Bruce Banner saving an alternate reality by redirecting a lethal blast of gamma energy. But in the process, he might have just accidentally dropped all that energy onto Asgard, setting up the Hulk's upcoming duel with Thor.
Hulk #6 seemingly pushed the titular Jade Giant to the limit, revealing his own brutal transformation into his Titan form. But the dark vision of Betty Ross who induced the transformation was caught off guard when the Hulk breaks loose of the engine room construct Banner had trapped him in. Banner was able to regain control of the Hulk just as he discovered his powerless variant warning him of a massive gamma bomb approaching his location. The ensuing blast would kill countless innocents even remotely near the site of detonation.
As the bomb approached, Banner was able to overload the interdimensional portal he used to come to this reality in the first place which allowed it to retrieve all nearby gamma signatures and absorb the fallout of the blast. Doing this saved countless lives, but it dragged Hulk and the energy elsewhere. Initially, they aren't sure where the Hulk and all that gamma energy will end up, until Hulk looks up and sees the Rainbow Bridge and an incoming Mjolnir. Previews have revealed that the next story focusing on both Thor and Hulk will be the Hulk Vs. Thor: Banner of War crossover, which will see the pair pitted against each other. The exact reasoning for their fight had been left unclear up to this point, but it now seems it'll have to do with Hulk accidentally dropping a ridiculous amount of explosive energy right onto Thor's home turf.
As the current King of Asgard, Thor would have to respond to such an event, even an accidental one, with a certain level of force. And considering it's the Hulk he'll be running into, it's likely that this accident to save lives in an alternate reality will just set off whatever remains of the old Avengers rivalry. Considering neither character is necessary in the most stable mental place at the moment, with the Starship Hulk story-arc revealing the disastrous new Titan form while Thor's battle with the fusion of the God Tempest and the Mangog resulted in the loss of his father Odin and saw Asgard almost destroyed it's unlikely any tensions are going to be resolved with compromise and understanding.
But it's a tragic turn of events that seems to be the impetus for their latest showdown. In the past, "misunderstandings" have been a common cause of conflict between Marvel heroes, with plenty of characters attacking the likes of Spider-Man or X-Men out of mistrust or a misunderstanding. Recently, conflicts between heroes, like the ones the X-Men have been sorting through with groups like the Fantastic Four, have become more nuanced and complex, with both sides having genuine reasons to grow concerned about the other. But it seems like a pure stroke of chance will deposit a devastating blast right onto the Rainbow Bridge, and Thor will discover the Hulk -- if not responsible then at least involved. Hulk vs. Thor: Banner of War is seemingly hearkening back to that earlier era of misunderstandings starting fights between heroes. But considering their own recently elevated power levels and the strain of their respective adventures pushing them to a breaking point, Thor and Hulk's duel could end up becoming far more explosive than anyone could have expected.
