were you revving it cold?
My brother blew 2 frost plugs out of my moms 93 Rolla by not letting it warm up. Coolant was thick and pressures were high. When he reved it hard, pop... out went a frost plug and the block heater.
Ive seen it happen on lots of cars that are abused in the winter, especially higher mile vehicles.
Just my 2¢
Aaron