I read this as javascript possibly being the language of recent growth: It's relatively young, very much en vogue, and so new startups are likely to use it disproportionally (because the guys who have been around for a while will not just switch their millions of lines of code to js because it's now become a thing).

But startups are also more likely to falter quickly in a recession than the old guard. And that's where java (or c#) is overrepresented.