insane attack for no reason and he fell asleep with his mind trying to make the moose have reason. 

Long after this incident, Brian continues to think of the moose attack as something crazy, something that could not be explained rationally by finding a cause for it. Even after he gets home and is able to research the things he didn't know during his time in the wilderness, like the real name ("grouse") of the creatures he'd called "foolbirds," he still doesn't know why the moose had attacked him.

Brian does accept, however, that nature's power is sometimes random and cruel. That's what he learned from the moose attack as well as the tornado that hit his shelter while he was still recovering from the damage the animal had inflicted on him.

Still, as readers, we're curious to understand why the moose did attack. Surely it wasn't just for fun, and surely the enormous moose didn't feel threatened by Brian's small arrow that he was using to find food at the time, right? The novel provides no answers. But if you Google why female moose like the one in the story do attack, you'll learn that they are extremely protective of their calves. If the moose in the story had her calf nearby, she might have attacked Brian even though he didn't provoke her. (Her intention may have been to demonstrate to Brian that he'd better not even think about getting close to the moose calf!) In fact, because female moose are known for being so protective and defensive, this discussion from SurvivalGrounds.com indicates that in Canada, female moose are responsible for more human deaths than even the grizzly bear.

It's Chapter 16 of Hatchet and Brian is washing his hands in the lake. He's suddenly aware of a strange presence behind him. When he turns round to see what it is, he's confronted by the sight of a large, angry moose. Brian may have grown skilled in hunting rabbits and trapping foolbirds, but a moose is a different ball game entirely.

Before Brian has had time to think, the moose attacks him, throwing him into the water and coming after him to finish the job. Brian is badly injured in his ribs and shoulders, but somehow needs to escape—and fast. This is truly a life and death situation, the most serious incident that Brian has faced since crash landing in the wilderness. The moose keeps driving Brian deeper and deeper into the muddy water, filling his lungs with filth, until, all of a sudden, she stops, giving Brian an opportunity to struggle to the surface.

But no sooner has he done so than the moose attacks again. Once more, she charges straight at him, sending him crashing back into the lake. And once more, she leaves him alone to continue calmly munching away on a lily pad root. Brian figures that the best way to escape is not to make any sudden movements that might antagonize the moose. So he gets down on his hands and knees and crawls very, very slowly away from the lake until he finds a place of relative safety behind a tree.

In Hatchet, Brian grows tremendously during his time in the wilderness as he learns to survive on his own. We can see this by comparing Brian's reaction to the porcupine incident and his reaction to the moose attack and the tornado.

When a porcupine attacks Brian and shoots quills into his leg, Brian collapses in a fit of crying. He doesn't think he will survive at this point, and he's not even sure he wants to survive. He is thoroughly miserable. But after he cries himself out, he realizes that feeling sorry for himself will help nothing. He just has to go on.

And Brian does go on, meeting challenge after challenge and getting better and better at coping with them. When he is attacked by a moose that throws him into the water, he climbs out, picks up his tools, and goes back to his shelter. He has learned to manage his pain and continue to do what needs to be done. That same night, a tornado roars through, destroying Brian's shelter and putting out the fire. Brian is not devastated like he was with the porcupine. Rather, he simply decides that he will rebuild, even as he swats the dozens of mosquitoes swarming around him.

In fact, Brian grits his teeth, looks out into the darkness, and throws up a challenge: “Is that the best you can do? Is that all you can hit me with—a moose and a tornado?” Brian then smiles, realizing the change in himself. He no longer feels like crying. He is “tough in the head,” and that is what will bring him through this ordeal.