I didn't say it was rare, just that I haven't seen it in a long time.
These things happen when conditions fit a pattern. Doesn't matter which ticker. Something with low volume, a spread wide enough to be exploited, and a catalyst on the horizon. Most traders are loaded already, and those who are on the fence see the declining PPS, but when they try to sell the offer they get shaved, so they turn around and hit the bids....the bids who have one tick shaved everyone to be on top.