It seems to me it could be one of 2 things.
1) yes maybe a freeze plug fell out and that will bring on water faster than you know what.
2) What really caught my eye is, when you cought air off that wake than came down then noticed the leak. That could mean you blew a bowl gasket. How that happens is the pump is still loaded with pressure then slaming all that weight can blow a bowl gasket.
Best way to describe it is take a drinking straw and a patato, put your point fingure over one end of the straw, and slam the straw to the patato, and it will go right through. All because of pressure.
Also if you don't have one already, get one. A flexable mirror, it's a great tool to use to see if you did blow a freeze plug, and other things you can't see.
OverKill