1. How about a nuke or energy deltahuman used to power the city, but hasn't had any agency on how they use their power because they're owned by the government/government experiment gone right (not public knowledge this person exist). They gain an awareness of how they're being used and is sick of it, so they charge up and release it all at once destroying the city, and maybe themselves with it. 

2. The government doesn't have power control programs and counseling to help children learn to control their power. Powerful abilities aren't rare, disastrous ones are. LA was devastated by children, twins, combined natural disaster abilities. Tsunami, tornado, earthquake, etc.