CK Vulpeculae: Something went boom but we don't know what

There are some objects in space that we kinda sorta know what they are, but the details are still fuzzy. For example, CK Vulpeculae is very obviously a bipolar nebula, made of gas and dust in an hourglass shape. We also know it formed in late 1670, when a star in the constellation Vulpecula (the fox) underwent a tremendous outburst of energy, going from invisibility to easy naked-eye viewing.

But... what is it? That is, what specifically caused the star to suddenly grow immensely in brightness ...