Anxiety, depression, desire to experiment with different drugs, hormonal and other biological changes, identity dysphoria seems to be in now and causes confusion in a young person. Grown ups are not secure and stable role models. They have their own problem. Hostility may be one side of anxiety too.
They do not have a proper outlet to deal with their negative emotions including anxiety, insecurity, etc. If parents and elders are not there for them listening and validating what they are going through they may turn to other role models like gang leaders, thus perhaps turning hostile and violent. Some kids find acceptance and belonging when they are part of gangs or other not-so-useful peer groups.
Yu ask the following: "Why teenage children turn to intransigence and hostility?"
1 Biological modification in the adolescence,
2) Cognitive changes. Adolescents are generally formal thinkers in Piagetian terms.
3) Formation of the sense of identity, which leads adolecents to affirm their sense of self by rebeling, for example, against theirs parents, and consequent entrance in gang groups.
4) Their dreams for a better world. In principle, an adolescent is a revolutionary in the sense that s/he wants a better world than that wherein s/he lives.
5) Needless to say, all of us have already heard fro adolescence' crisis, which lies at the heart of intransigent and hostil behaviory on the part of the focal adolescent.
Many who study this period of human development are still influenced by the "deficit approach" to the study of adolescence, which arose in the 1950's and seemed to reinforce past assumptions of the teenage years as a time of "storm and stress" (per G. Stanley Hall). It was anchored in a prevailing mindset of developmental deficits, with a primary focus on prevention and remediation of risk behavior in “broken” individuals. However, the research that formed the base of this approach was conducted through convenience samples of youth living in juvenile detention facilities, mental health facilities, and the like. Much of the research was quite good, but it was incorrectly generalized to the larger population. Prior to the mid-1970's there were no systematic empirical studies of adolescence within a family context at all, according to Steinberg (2001). There are a number of strengths of adolescence that this approach failed to recognize. For a more accurate approach to the study of adolescence, I suggest a dive into the more modern approach of Positive Youth Development (PYD), which has arisen in the last 20 years or so.
There are normal mood swings, as adolescents come to grips with the intense feelings resulting from the new hormones surging throughout their bodies. It's also developmentally appropriate for teens to prefer their own solutions to those of adults, as their brains take on new cognitive abilities in a transformation process more profound than at any time since toddlerhood. However, despite all this, contemporary research shows that the majority of teens have positive relationships with their parents and families.
Teenagers look for their way, their looks, their style, and so on. This search goes through breaking the boundaries expressed in irritability, hostility, sometimes depression, and so on.