The cap was probably installed backwards, reversed polarity. When this happens, everything seems normal, then you begin to hear a sizzle, then the cap explodes spewing insulator and electrolyte. Clean off the board with iso alcohol or Castrol super clean to make sure the electrolyte is removed. It is conductive and can cause other issues.
How long does it take an electrolytic cap to explode? That is dependent on the voltage applied, capacitance and size of the cap.
--
Chris Hibler - CARGPB #31
http://www.ChrisHiblerPinball.com/Contact
http://www.PinWiki.com - The Place to go for Pinball Repair Info