This is just an opinion, but I see several factors. I don't think it's about professional equipment, but I do think it's about more expensive gear.

Today's point and shoot cameras are very, very capable, and it's hard to point out obvious things that spending more money will get you. So, when your friend asks what your new $700 DSLR can do that their half-that compact can't, and they're not too impressed about interchangeable lenses, you say "ahh, it can do this beautiful, creamy background blur; that's what I'm getting for my money". Or maybe you don't say this to your friend; maybe you say it to yourself when justifying spending a month's salary on a full-frame DSLR. (I'm not judging, you understand. I'm pretty solidly in the photo-spending rationalization boat myself!)

Second, it's a pretty easy effect to do. You don't really need special skill or to spend a lot of time or careful effort. You get the right gear and you point it right, and, bam, you've got blur. That's pretty appealing in a trend, because there's not a big educational, skillful, or artistic barrier to entry.

Third, I think digital has spawned golden age of popular photography. There have always been photo enthusiasts, but we're at a whole new level. My grandmother had an SLR, as did my dad. They didn't just point-and-shoot, but they weren't really concerned with making artistic photos; they just wanted nice-looking ones and didn't spend a lot of time (or money) experimenting. Digital means people can experiment more, getting instant results and with reduced per-attempt costs. That's not a reason for this in specific, but it's a reason that something that has been a thread in enthusiast, artistic, and professional photography all along is now a big deal.

And finally, sometimes things are just trends. People like a look and copy it.