The window for booking SO exchanges for summer 2016 hasn't opened up yet, so it can't be related to competition due to banking. Nor can it be related to partial weeks because nobody, even owners, has the ability to book a partial week at this point in time.
My guess is that it has to do with in-season competition. Starwood designated "platinum" season at HRA as weeks 18-34, or the first of May through mid-August. However, weeks 25-34 (2nd full week of June to mid-August) are when most school kids are out for the summer, and those weeks will always be much more popular than May. (And a quick search of the OnLine tool confirms that.)
FYI, this is hardly a new phenomena in the TS industry. I read TUG that there are Marriott owners who actually buy a second "junk" week, just so they can book at 13 months out at their target home resort, and get a jump on single-week owners, because competition for summer weeks is so fierce. (Multi-week owners can book at 13 months instead of 12 months.)
I think Starwood owners lucked for many years because the system was in its infancy, and because the clunky call-in reservation system veiled how popular certain weeks were over others. As time goes by, I imagine it'll get even more difficult to book one of these prime weeks because more and more people, owner and exchangers alike, are catching on.
Personally, if I was chained to a school calendar, I wouldn't rely on exchanging in during the summer, unless I wanted to exchange to a resort where summer IS low-season, such as Colorado or a desert property. I'd own where I wanted to go most places. At HRA, I'd even consider buying a fixed week because exchanging is difficult and rental prices are astronomical.