It makes sense that they're starting to cut their hair at the moment. Since it's summer in Japan, long hair probably gets really hot and heavy because of the heat. I'd imagine it also gets in the way during rehearsals/lives -- even more so than usual. Less hair = less sweat, most probably.
I'm just speaking from personal experience, because my hair gets really gross during the summer (I grow it out in the winter, so it's usually long by summer time) and I always end up cutting it before it gets too long. I don't do any kind of activities like idols do, but I assume it's even worse for them. So... I totally understand why they'd want to cut their hair.
People complain about it, but it's a lot more convenient having short hair, so, y'know. Also, hair grows back.