A: Sibling rivalry is alive and well in most families with young children. So don't be surprised about your older son's actions -- it's pretty typical and expected. Just think about how he feels -- he had you all to himself, and now he must share most every waking moment with a noisy, demanding interloper. And he's too young for us to expect he'll be "mature" enough to accept his baby brother gracefully -- OR that he'll be able to "explain" his aggressive behavior to you. Those are all very advanced emotional processes that will take years to develop -- with your help, patience, and understanding.
In the meantime, it's up to us as parents to protect the younger child while the older child is around. Be matter-of-fact and calm about it, and don't make your older child feel ashamed for his aggressive desires -- but DO let him know you can't allow him to hurt his baby brother. Heap praise on your older boy when he plays nicely with the baby -- even if it's only for a few moments -- and point out how much the baby looks up to his big brother. Also, the more you keep your older son active and entertained with age-appropriate peers and activities, the happier everyone will be!