Why is aluminum wiring dangerous?

“Single-strand aluminum wiring expands and contracts more than copper when current flows through it. Over years, this causes connections to loosen at outlets, switches, and fixtures. Loose connections arc and generate heat, which can ignite surrounding materials. The CPSC found that pre-1972 aluminum-wired homes are 55 times more likely than copper-wired homes to reach 'fire hazard conditions' at connections.”