That’s just two weeks after the Batman meets Superman movie, which arguably shows some real brass on the part of Marvel.
Perhaps more tellingly, it moves Ant-Man out of the way of the next James Bond film. Edgar Wright has made more than a few references to his Ant-Man picture having heavy spying and espionage elements.
The easier move might have been to push Ant-Man back into 2016, out of the maelstrom of 2015. That Marvel didn’t do this highlights the deliberation with which their releases are ordered. As standalone as Ant-Man might be, or might initially appear to be, I doubt it can just be shuffled out of sequence without losing a little of its edge.
None of this should effect the project’s shoot date too much, with Ant-Man still set to go before cameras next spring.