Garlic resprouting

As you've discovered they will resprout if left in the soil after dying back, and next year each clove in each bulb will try to grow into a new bulb if you leave them where they are.

I proved this accidentally after somehow managing to leave an entire garlic bulb unharvested - the cloves on the resulting bulbs the following year were not as big as they would have been if they'd been planted normally (i.e. with a lot more space to grow  ) but they did produce usable cloves.

Not much use for culinary purposes now they've sprouted - you could try separating the cloves if you can do that without damaging them and replanting them individually at the proper spacing (Voila - autumn sown garlic!)

I have always heard to plant garlic on the shortest day of the year, harvest on the longest.

My latitude is a little off, but I think that is what your garlic is trying to do naturally.