This has been a highly visible trend over the last year or so. On Match, for example, there's a lot more boobage and visibly hard nipples than there used to be even though the user base is more or less the same.
There's been a lot more boob-glamorization in the media lately, imo it started with Chelsea Handler and Madonna splaying their boobs all over the gossip pages and now it's snowballed and drifted down into the younger age groups.
There is another aspect of this phenomenon, in that is that it seems like they're running out of things to overturn in their Inversion Agenda, so lately they've been sexualizing older women. And many of them have a nice rack as the only thing going for them.
IMPORTANT NOTE: This is only a TREND.
Just because you see a woman IRL or online braless or with their tits out, it doesn't necessarily mean that they are more DTF.
They're just following the trend, so don't get fooled.