In the last week or 2 - well after TBB 9.0.9 release, StartPage quite
often shows "sorry to interrupt" page, asking if you're doing weird things.
Has a place to send comments, explanation. An excerpt (emphasis mine):
Just Checking... We apologize for the inconvenience: to prevent
possible abuse of the Startpage.com service, your Internet connection
has been prevented from accessing it at this time.
This happens when a *large number* of search requests are received
from one's Internet connection in a *short amount of time* -- for
example, if you are using "screen-scraping" software, or if you are
sharing a connection with many people, perhaps through a proxy service.
It's not tied to exit nodes in certain countries, AFAIK. As with
youtube, quickly trying new circuits 5+ times has mixed results. I
imagine they've recently had bad experiences or changed policies.