What people are running into is an anti-crawling measure that I always hate having to turn on.
The intent is for all public Pinboard pages to be visible without a login. However, user+tag pages in particular are somewhat expensive to generate on the fly, so a crawler that iterates through them can quickly bog down the site.
In the past, it was fairly easy to block or throttle this kind of crawling. But in recent months, I've seen a rise in distributed crawling from China+Singapore IP addresses, with no patterns that would make the traffic easy to block. So I'm forced to either put pages behind a login, or have the site become unusably slow for everyone.
If I have to I'll block the entire PRC address range, though I'm looking for alternate solutions that are less drastic. But I just want people to know the reason for the back-and-forth behavior on public pages.
Maciej