It was (probably) capitalized in the original submission. But Hacker News's title autoformatting screwed it up, again. It's known to capitalize individual words but removing all-caps acronyms.
It's the annoying part. The anti-all-caps-clickbait system knows some common abbreviations, but not others. For example, on multiple occasions, "MIT" was not unaffected but "CSAIL" was turned into "Csail", see [0].
There is definitely some find-and-replace code. After you click the submission button, the title is immediately "corrected" (submitters can manually edit the title back later), with good intention - anti-clickbait. It basically does three things: remove superfluous words, remove superfluous caps, and properly capitalize the title. Most of the time it works well, but annoying when there's a false-positive.
Examples I've seen so far (note that it's not always triggered, there must be some "if" conditions here. It's not always reproducible).
* All lower-case titles are capitalized.
Generally good, but not always enforced, it's strange.
* Unnecessary caps are removed.
Many false positives. "MIT CSAIL" becomes "MIT Csail", DARPA becomes "Drapa", etc.
* "%d Ways To Do X" and "How To Do X" becomes "Do X", per Guideline.
It misfired when I tried to submit "20 °C – A Short History of the Standard Temperature for Dimensional Measurements" by NIST. The "20" was removed and I had to edit it back!
A submission of "How we threat model" by GitHub became "We threat model", makes the already-short title unreadable.
* Clickbait words are removed.
Generally good. But false positives exist. For example, "Massive Parallel" is a legitimate concepts in computing, but the word "massive" will be removed. I just tried "Massive MIMO", which is similarly a legitimate concept in communication, got removed as well.
Try submitting this paper "Pilot Optimization and Channel Estimation for Multiuser Massive MIMO Systems" (https://arxiv.org/abs/1402.0045), you'll see that the word "massive" is deleted immediately after submission (but as I said, the reformatter is not always triggered and not always reproducible, perhaps your high karma will stop the filter from doing it).
* "Your Statement is 100% correct" becomes "Statement is 100% correct".
This is a good one, the unnecessary personal element is removed.
Thanks for the detailed examples. Maybe it's karma threshold, maybe a bug, or maybe I never wrote a title that would trigger the autoformatter, but I never noticed it.