What causes blue copper acetate crystals to form in a vinegar solution?

[Paul77]

So I was trying to make a simple copper acetate solution by letting vinegar sit on some copper scrubbing pads, and after a few days I noticed these tiny, perfect blue crystals forming at the bottom of the jar. I wasn't really expecting that, and now I'm just curious what exactly makes them suddenly start growing like that instead of just staying in solution.