From a
www.jalopyjournal.com thread titled "Why are suicide knobs called so?":
"There are a few reasons: One, they will sometimes grab a shirt sleeve and get snagged up right in the middle of a turn. Two: in the case of bump steer, they'll just about knock your thumb off and shove it down your throat for ya. Three: They've been known to pick the worst possible moment to break off, like right in the middle of a hard turn. None of those scenarios is very pleasant."
They also seem to be known as brodie/brody knobs and spinner knobs. The thread contains various accounts of people breaking thumbs and wrists on them!