I literally just replaced mine tonight. The seam were the top mold meets the bottom mold separated allowing a mist of coolant to spray out once under pressure. I Pulled a replacement tank from my donor truck. Hopefully, this one holds up.

 

Wow, I can't imagine that happening! I would assume that it was due to extreme hear or over pressure of the tank.

 

i can believe it. the seams on mine seem to be spreading and have micro cracks in some spots. looks like only a matter of time before it lets go if the pressure doesn't find another week spot, i.e. the throttle body nipple.