My SWAG -- Scientific Wild-A$$ed Guess -- is that in the early days of pipelining, the root was run fairly 'cold' and fast. This closes up the open gap in the root. Then comes a hotter, slower pass to tie everything a little better: the Hot Pass.
If you watch any root when the 2nd [hot] pass is being welded, it does indeed get very hot, and portions of the root may remelt.