In what years did the United States have the highest fatalities? how many? why are the numbers so high?

Well the deadliest disaster in US history was in 1900, from the tropical Galveston cyclone in texas, which caused 6,000-12,000 deaths and around $28,000,000 in damage. Only 6 years later, the second deadliest disaster was the 1906 San Francisco hurricane and fire which claimed over 3,000 deaths.