This is the second time in a week someone has used “tumble” to mean “occur rapidly” instead of “fall”. Is this a new colloquialism or had"tumble" always had a second definition as “occur rapidly”?

If a condition is worsening (a “fall”) “tumble” applies just fine. Indeed, “tumble” is just a way to say “falling rapidly” in this context.

The reason “tumble” (and its notion of “fall”) is applicable is because the situation is worsening. If it was rapidly improving, nobody would say “tumble”; it’s not simply that it is occurring rapidly.