Typical US water heaters have two elements, upper and lower; each 1.5-5.5kW, with a double-throw thermostat that select one or the other. What may have happened was both elements were energized simultaneously, which generally is not intended.
Example see page 1 “non-simultaneous operation”
http://www.hotwaterproducts.com/BWC/Parts/Pdfs/Electric%20Wiring.pdf
