Gorillas have fear when it comes to challenging there authority. This is one of the major reasons why gorillas would fight.
When a male gorilla grows up may be about 13 years, it will either convince some of the members of the family to join him form another family or fight to topple his father and become the dominant silver back.
In another scenario the dominant silver back may feel too weak to protect the group anymore and so it will choose one of the young black backs to assume leadership.
Gorillas have sharp and strong teeth. This facilitates their fighting as well. The fact that they has something or a weapon.
Male gorillas are a dangerous opposition to fight. Gorillas also have differences in opinion and sometimes they have to be resolved by a fight. In most cases no one will be hurt or injured; it is a matter of showing or proving who is great or stronger.
Gorillas are well known for the famous behavior of beating the chest and loud hooting. This is more impressive with the silver back.
There is a lot of aggression in big groups and some are also calm but when two groups meet, the dominant silver backs can engage in a fight which can even lead to death and one may want to take over another group or for any other differences they may have like invasion of territories.
There are always lots of injuries and severe ones in this case.