Neglect, really. I let the biga go for the maximum time (3 days) but didn't get back to it until late at night. When I realized it would need many more hours to make into bread, I mixed the ingredients, let it rise and then put it back into the fridge. Tsk-tsk. Needless to say, it was beyond repair the next day, and certainly no longer seemed worth it. What's up with my slacking off?