The link between anger and anxiety is an under-researched area, but in our work, the manifestation of anger in anxious children is clear. Here are some hypotheses as to why there is a link. Anxiety occurs when there is an overestimation of a perceived threat (e.g., a test or a party) and an underestimation of coping skills (e.g.,"I can't handle this."). When our kids are chronically and excessively worried and don't feel like they have to skills to manage the anxiety, they feel helpless. Helplessness leads to frustration which can show up as anger.
Anger and anxiety are also both activated in the threat centre of your brain. When the brain perceives a threat, the amygdala (a small, almond-shaped cluster of neurons in the brain) activates the flight-or-fight response which floods your body with hormones to make you stronger and faster. This genetic wisdom protects us from threats and danger. Because anger and anxiety are both activated from the same brain region and have similar physiological patterns (rapid breathing, heart racing, pupils dilating etc.), it's possible that when your child feels like there is a threat (e.g. going to a party), the fight or anger response is activated as a form of protection.
Finally, one of the markers of generalized anxiety is "irritability" which is also part of the anger family.