Overproofed!
Over-proofing is often blamed for collapse in the oven. This is not always the case, as will be illustrated in this posting. A fully developed dough with good gas retention properties will not collapse in the oven, but will instead grow enormous in size. Baking this loaf was an experiment in extreme over-proofing, and an investigation into the causes of collapse in the
Source: thefreshloaf.com