Exactly, Robbie. Anxiety disorders, which come in myriad forms.
There's a whole shit more to it than a 'temper tantrum'. A common thing that I've noticed is that other adults think the the child is 'acting up', when in fact they are out of their comfort zone and feel really scared. I can remember similar things happening to me as a child. For example, my grandmother wanted to go to the cemetery and visit my granddad's grave, but I'd never been to one before and felt terrified of what would happen, and of the sadness of the adults around me. So I got upset and I probably looked stroppy and entitled, so we didn't go and everyone told me how selfish I was. I hated myself for that for so long after.