It isn't a reflection of what's popular, it's a reflection of the media and major festivals (not just G) adjusting to what's popular.
If you look at the charts in the mid-2000's when the festival was indie-heavy, you'll see that they were still dominated by pop music (albeit with a week from Ting Tings or Kaiser Chiefs here and there) even if the post punk revival was going on. It's just that was happening at major festivals and music publications and now it is not.
Pop music hardly got bigger or better, but it did gain more credibility over the years as the taste-making publications and brands had to go online and rely on clicks and likes rather than a sheer following. Similarly, rock music hardly got worse or less popular, it's just that the festival is booking for a more populist audience now than the in-market festival fans it used to.
Festivals used to be the refuge of the rock fan, and those fans still go to Glastonbury because it attracts a loyal fanbase who don't stop going easily. But, you do have festivals like V Fest, which died chasing the pop crowd. A noticeable shift in booking policy is likely to rankle some of the older Glastonbury audience and it's no surprise that the mid-sized festivals catering more to the disenfranchised are getting more popular year-by-year as the majors change their tack.
I can't imagine that the resale was actually easier this year and it's purely anecdotal; definitely way too early to say. However, it's something to keep an eye on as the years go by. If people gradually decide that Glastonbury isn't for them anymore and the crowd that the festival pivots its acquisition focus towards decide it's too expensive and cheugy to bother with, the fest could struggle to sell the entire allocation of tickets instantly one year.