While large-scale flash mob thefts have been on the rise this year, experts say they hit critical mass late last month, when stores were piled high with holiday inventory. On Black Friday, a crew descended on a Home Depot in Lakewood, Calif., making off with sledgehammers, crowbars and hammers. The same day, roughly 30 people swarmed a Best Buy near Minneapolis. A Bottega Veneta boutique in Los Angeles also was hit that day.