If you want the gory details, kneel is a Class 2 irregular verb; there are about fifteen or so verbs that have an -ed past ending that is differerent from the participle form of the past.
Burned burnt, or spoiled spoilt, send sent, smelled smelt.
In Old English there's already a pattern that associates a preference for t or d with a dialect. In modern English, British English generally goes for the t.
In American, well, it depends on the speaker. Most publishers have a list of preferred forms in the House style sheet, and editors will follow it. Authors who don't like the preferred form use a stet and everyone ends up, well, if not happy, at least not morally outraged.