coronavirus pandemic and street protests over the police killing of George Floyd, exhausted cities around the nation are facing yet another challenge: a surge in shootings that has left dozens dead, including young children.The spike defies easy explanation, experts say, pointing to the toxic mix of issues facing America in 2020: an unemployment rate not seen in a generation, a pandemic that has killed more than 130,000 people, stay-at-home orders, rising anger over police brutality, intense stress, even the weather.