What's going on at the northeast corner of the arena block? The removal of construction trailers means...
There was no explanation, but it's safe to assume that, with the near-completion of the arena, those construction trailers were unnecessary/redundant. There are no plans yet to build on that site, aka B4.
It is slated to the the third of the modular residential buildings around the arena--first, the 32-story B2, which began construction in December at the corner of Flatbush Avenue and Dean Street, then the somewhat shorter B3 at the northwest corner of Sixth Avenue and Dean, then B4 (about 50 stories) at the southwest corner of Sixth and Atlantic Avenue.
I think it's safe to assume that the developer want to make sure they can get the first two buildings right before trying one that's much larger. B3 is supposed to start six to nine months after first is completed (which should be mid/late 2014), and third building on the same schedule, so that suggests an early 2016 start for B4. But nothing with Atlantic Yardsis predictable.
I assume the removal of the construction trailers will be discussed at a future public meeting, such as the next Atlantic Yards Quality of Life meeting, likely in late April.