Granted that I'm not a pro rider, and while I did race NORBA in sport class for years I've never competed in a road race. That said, back in 1992 when I first started getting serious about riding I was taught to try to hold a 90 cadence on the road and to this day that's where I run. If I ride a long reasonably flat ride my 305 will show an average cadence of about 88 or 89. So if 90 is high then it's surely not a new "fad".
I did have to learn to lower my cadence for MTB riding. I rode on the road for a long time before I started racing MTBs. On a MTB you can't sit the saddle and spin, you need a cadence that is low enough to let you pop up to float over bumps, but on any long climb I still sit and spin at about 90 if I've got a low enough gear that my 49yo legs can push it. In my experience pushing bigger gears with a slower cadence on long climbs causes lactic acid build up based on the how bad it hurts meter.
I got into single speeding the last few years. I use to sit and spin away but the SS taught me to get out of the saddle and work the bike, but conversely it also taught me to sit and spin because you only have one gear. It is a great training tool and makes you a stronger rider and faster when you get back on your geared bike.