Sounds like simple shrinkage. the center of the liquid weld cools slowest, (impurities with lower melting temperature also migrate there), solidifies last, and as it does - contracts - pulls inward on surrounding solidified metal. a little like a pour basin in casting, piping in an ingot. When you get TOO much shrinkage TOO fast, then you get a crack.
In explaining solidification of a weld I often describe it as the same as water in an ice cube tray- the center solidifies last.