The writer of this sentence is being clever. They have written something that isn't straightforward English. They think it's amusing because the words "spending" and "trending" rhyme. A "trend" is a movement that fluctuates up and down a bit, but overall, progresses steadily in one direction. "Trending" probably means trending _upward._ You can't answer the question unless you know exactly what kind of "spending" they are talking about. For example, younger people use their cell phones more, buy more expensive phones, and use more cell service. So it's possible that cell phone spending might be "trending" because of demographic trends.

It is not common. I can say what I think it means. "Why do you think spending more money on things like enjoyment, fancy cars, or jewelry be more popular now than it has been in the past?"