I Believe This is HN's Auto-Capitalisation of Titles.
It really annoys me when submitting; it does it only if there's a certain number of capitals in the submission, so 'ARM' probbaly triggered it. (It's now 'Arm' anyway, so it needn't have.) It does accept a more sensible case on re-submission if the submitter edits it quickly enough.
"ARM-Ed" sounds like a special ARM architecture, while ARM-ed and ARMed elicit the author's intended meaning to me. It's pretty clear as to why the author didn't capitalize "ed" in their own title.