We have a seasonal business cycle that has been going on for 100 of years.
There are a lot of theories.
I personally believe that every 5-8 years clusters of business mistakes accumulate until losses amount at an macro-economic aggregate level that can no longer be sustained. Before the Central banks, the business cycle moved up and down more frequently--- every 2-5 years.
One example of a business mistake cluster--- Basically, a few people make more than normal profits doing something, then everyone jumps-in, but there really isn't enough room everyone, so lots of people take losses and fold. I have seen so many examples of this in my life that it's hard to count. One of the most clear-cut examples of this was the Asian Yogurt shops on the west coast. There were so many different players at one point. Now, there are only a few remaining. The winners were the Company's with the lowest prices, best flavors and best customer service. If you watch Asian communities, you can really see these trends because they are so business minded. One year, it's Yogurt, then it's Southern creole food, then it's shipping recyclables to China. Then it's European inspired bakery shops. It all happens in the same pattern, there is one-shop, then there are dozens of different brands, then there are only a handful left. We are normally left with lower prices for the highest quality.
At a global macro-economic level, I think there are a few business mistake clusters that should be cleared out by a recession. One is the Chinese economy. There are a too many State-owned enterprises that have made a lot of business mistakes. The whole state-owned enterprise segment of China needs to be seriously reduced. The second is the Social Media/App/Search industries in America, and this is most scary.
Social media/apps and search markets are dominated by a few players right now, but there are so many start-ups that make absolutely no money, and continue to make no money at all and don't ever intend to make money (see Snap S-1). What is scary is that Facebook, Google, and Amazon, make an insane level of profit, yet they are never the target of the Political left because they have gone to bed with the progressives. The normal push-back against the mega-corporations no longer exists. Walmart and big-oil, the traditional foes of the political left, don't really bring the type of money that Google, Facebook and Amazon do. However, even those these 3 companies make 5 times the level of profit as Walmart and Big Oil....and even though they really make their money by selling our personal information, which is far worse than Walmart and Big-oil, the Political Left so far has refused to attack them.
When the recession happens, and it needs to happen, Google, Facebook and Amazon are going to gobble-up all of their smaller competitors world-wide and even more probable, they will acquire companies outside of their industries. Simply because they are sitting on enormous piles of cash and balance sheets that can easily demand financing. We are going to live in a world dominated by 3 companies mainly because the progressives won't push their army of union workers and college kids to check these big corporations. Unfortunately, every college kid and every union worker would do anything to work for these companies, and they all enjoy their products. We really don't see the danger that is coming. The progressives failed us just as much as the conservatives. When we all have google TVs, and hail cabs run by Facebook-drive (after Facebook purchased uber), and get our Rx drugs at Walgreens (now owned by Amazon), and each new subsidiary becomes a new avenue to take personal information and sell it to advertisers and governments, maybe then we would have realized our mistakes.