The clay was over-fired. Pugging it had nothing to do with the result.
Your shelves might be salvaged . . . depends on how deep the clay melted into the shelf. Wearing appropriate safety gear, remove the items and grind down the shelf -- use an angle grinder if the clay did not melt into the shelf. If nothing else, remove/grind away all the melted clay and kiln wash to get the shelf completely clean, then flip it over an use the other side instead. But you won't know until the charred remains are removed.
Is it possible you programmed the kiln to cone 6 instead of 06? Did the kiln turn off properly at peak temperature? How long was the firing?