Sodak, I believe what you're describing is explained towards the very end of the webpage that Pete linked to--the sudden introduction of still-cold/cool blood from the extremities into the core circulation. The heart is still not really recovered and the sudden shock of more cold blood causes cardiac arrest or other cardiac abnormality. Very insidious.

It's similar to the danger faced by people pinned under heavy objects, like in an earthquake, through "crush syndrome". They victim will seem fine when they are initially rescued, can walk around and hug people, then shortly thereafter, just drop dead. It's a different mechanism, but it's equally terrible because the victim seems fine at first.