We always knew BvS was going to move, I just assumed it'd be to July instead of up to March, if not April. That said, the summer movie season has slowly been expanding/evaporating, and I think this move officially signals the beginning of an era in which the absolute biggest blockbusters can now start opening in March. March/April had no doubt already been home to mega-hits like Alice in Wonderland, Fast & Furious 6 and Captain America 2, but those were all still somewhat "second-tier" in terms of brand recognition. This move, however, now allows the Star Wars and The Avengers and The Avatars to start claiming early spring dates as well, if not other calendar dates outside of the summer and December that aren't in January, February, and September (the three months that still seem "off-limits" to mega-blockbusters).
Still, this feels weird, if not inevitable. But it makes sense, I guess. FOUR of Zack Snyder's previous movies have all released in March (Dawn of the Dead, 300, Watchmen, and Sucker Punch), so he's obviously comfortable with that time of year.