As I've always known it, Pop music generally starts out slower and sadder at the beginning of a decade, then picks up steam and gets more upbeat towards the end of a decade. You could trace this trend all the way back to the '40s, but I'm going to start here from the '60s. Then it all becomes clear.
In the beginning of the '60s, there were a lot of ballads, then Beatlemania in the mid-'60s, then the dawn of hard rock in the late '60s.
The '70s started with a lot of introspective singer/songwriters (James Taylor, Carly Simon, Elton John), then gradually the tempo went up to the disco craze of the late '70s.
In 1980, the disco bubble burst and soft rock and arena rock ballads ruled. Then came New Wave synth pop and later, hair metal, which dominated the late '80s.
In 1990, the hair bands were singing far more ballads than actual rock, then came grunge, which was about as sad and alienated a rock sub-genre can get and mainstream pop music itself almost vanished completely between the divided camps of grunge and hip-hop (or was regulated to Adult Contemporary radio) Pop returned in in the middle of the '90s and the decade ended with rap-rock Nu Metal.
Again in 2000, the Nu Metal bubble burst, then came 9/11 and ballads were once again the order of the day. But as before, the tempo increased and AutoTune pop wound up finishing the '00s
While AutoTune pop is still heard, again, we're moving back into ballad mode. But things will get more upbeat by 2015 and we'll wind up this decade with another pop music fad. And we'll repeat the process again come 2020-2021.
It's nothing new to me, that's just the general algorithm of pop......