Meh. It's a pretty standard Studio trick. If I recall correctly, Warner Brothers tried to do something similar by attempting to put Batman versus Superman in the same weekend as Civil War. Sometimes it's just a matter of two Studios facing each other down over a date and see who blinks first.
Actually, Disney had claimed the first May weekend, which is the most lucrative release date of the year, for years with Marvel movies, and did so well in advance. The first May weekend of 2016 was set for the release of a Marvel movie as far back as 2013 (and they set another unnamed Marvel movie for May 5, 2017, which turned out to be GotG2, at the same time), back then people expected it to be Doctor Strange or Black Panther.
WB obviously was (and probably still is) still pissed about Marvel claiming that release date every year, so announcing BvS for the same weekend was a strategic move to challenge Disney's claim for the best release date of the year every year. If it had been Doctor Strange or Black Panther, they would have stuck with that plan and Marvel would have changed the release date. But Disney didn't want to do so, and instead of a movie starring a new hero, they went with the 3rd Captain America, coming on the heels of the very successful Winter Soldier, and soon after to be announced to be co-starring Iron Man (after IM3 broke the billion dollar mark) and several other MCU characters, and introducing Spider-Man into the MCU.
So, basically, Disney defended their claim on the most lucrative release date of the year by one-upping BvS on several fronts. And since the MCU was so well established, they could risk one bad first weekend, while WB had their whole DC movie franchise set on BvS. Ultimately, WB had everything to lose, and that's why they ultimately changed the release date.
But Disney has a pattern of trying to spoil the success of competing franchises like that. When BvS was announced for Good Friday of 2016, suddenly that was the weekend Netflix would release the 2nd season of Daredevil. When Avatar 2 was set to be release in December 2017, Disney pushed Star Wars from its original summer release date to December. WB's marketing for JL and other DC projects was fought by whenever they'd make an announcement or release new material, Disney/Marvel would follow with something of their own in a matter of days (to the point that Kevin Smith even spoke about it in his podcasts), and this weekend will not only see the release of JL in theatres, but also of The Punisher on Netflix.
You can debate how effective they are, but at this point, it's pretty clear, Disney is a big bully, and not just doing "standart studio tricks".