How exactly do modern computer language come into being and why? How do they get their start and who is behind them?
It's a multistage process:
Pointy-headed type theorists and other professionals are continually proposing new language features. You can read about them in places like the Proceedings of the ACM Symposium on Principles of Programming Languages (POPL), which has been held annually since 1973.
Many of these proposals are actually implemented in some research language; some research languages I personally find promising include Coq and Agda. Haskell is a former research language that made it big. A research language that gets 10 users is often considered a success by its designers. Many research languages never get that far.
From research to deployment I know of two models:
Model A: A talented amateur comes along and synthesizes a whole bunch of existing features, maybe including some new ideas, into a new language. The amateur has talent, charisma, and maybe a killer app. Thus C, Perl, Python, Ruby, and Tcl are born.
Model P: A talented professional make career sacrifices in order to build and promulgate a new language. The professional has talent, a deep knowledge of the field, and maybe a killer app. Thus Haskell, Lua, ML, Pascal, Scala, and Scheme are born.
My definition of a professional is someone who is paid to know about programming languages, to pass on that knowledge, and to develop new knowledge in programming languages. Unfortunately this is not the same as designing and implementing new languages, and it is not the same as making implementations that many people can use. This is why most successful programming languages are designed and built by amateurs, not professionals.
There have been quite a few interesting research languages that have had hundreds or even thousands of users but yet never quite made it big. Of these one of my favorites is probably Icon. I have argued elsewhere that nobody really knows why languages become popular.
Summary: Languages come into being because people want to make programming better, and they have new ideas. Languages get their start when somebody takes a whole bunch of ideas, some new and some proven, and synthesizes them into a coherent whole. It's a big job. The person behind a new language might be a programming-language professional, but historically, most languages that become widely used seem to have been created by talented amateurs.