Best Buy CEO Brian Dunn abruptly resigned on Tuesday as the once-dominant electronics retailer struggles to cope with onslaught of digital competition from Amazon and other e-commerce sites. The company said in a statement the resignation was unrelated to any disagreements between Dunn and the board “on any matter relating to operations, financial controls, policies or procedures.” Some analysts cheered the move. Best Buy had been hit hard recently by “showrooming,” in which customers use smartphones to comparison shop for goods online while in the store. What has really crippled Best Buy, however, has been the collapse of its entertainment sales, made up primarily of DVDs and video games. Comparable-store entertainment sales plunged 21 percent in the most recent fourth quarter, after falling 14 percent in the previous fourth quarter. DVDs and video games had long been critical traffic drivers for Best Buy. But as sales of physical media declines, so too did traffic to Best Buy and other brick-and-mortar entertainment retailers.