It's an unfortunate use of term, because while fizzy does mean bubbly and frothy, it can also mean unsustainable because the fizz in drinks never lasts.
Author should have / could have used the title "Tech Investors can't get enough of Europe's growing startup scene". Would have conveyed a much less bubbly / frothy / transient nature.
My mind jumped to "fizzling out" before the "fizz" = "frothy" or "fizz is just temporary" associations kicked in. This is definitely a poor choice of adjective.
They have to say fizzing because not all of the startups are in the Champagne region of France.
In this context, it's an allusion to carbonated drinks and is a different way to say 'bubbly.'