It's called a labor market. Currently the demand is outstripping the supply (more demand for developers than there are developers) and so the cost of the product (labor) is rising. The rising costs (wages) should encourage more folks to become developers and thus stabilize, or even lower, the wages. But as it turns out, it's not that easy to become a good developer so it's been difficult to increase that supply to meet the demand and this has been the case for forty years now - so I wouldn't worry about this being a "bubble" that's about to burst.