Is it because I didn't wait
for the checks to complete before merging?
(It's a documentation only change that I previously committed to
main without incident, so I didn't feel the need to wait for all
of the checks to run before merging.)
While this is somewhat off-topic, I'd like to encourage you not to skip required checks on backport branches. We could turn them off if they weren't important, in fact quite a few checks are only running on the main branch. We leave the rest because backports can easily regress due to differences in untouched code. Documentation changes specifically can fail if references between branches change or doctests don't have necessary imports. You can also trip up a bug in Sphinx 2.4.4 (used by Python 3.9 and 3.8) that is already fixed by Sphinx 3.2.1 (used by the main branch and 3.10).
Don't get me wrong, I'm sure you'll use your best judgment. I don't intend to police anyone, just wanted to highlight that even innocently looking doc changes can break us every now and again.
I understand that you might not have much free time to monitor backports. It's fine for you to just leave them, the release managers will sweep them periodically.