That's basically what I was suggesting with the other possibility, although you've explained it more depth: "a completely random increase by really bad speculators (relatively unlikely I think)"
It is certainly a possibility, but it usually takes coordination action to get real price movement to happen in such a case. That still doesn't mean they are pumpers, they just might be a group of bad investors acting together, but the selection of HBD as the coin to invest in, which receives no real advertising and has very small volume, makes me believe it's an intentional pump attempt.
Spontaneous action can drive further spontaneous action in a positive feedback loop. I don't think it is unlikely, I think it is the norm in markets that are highly disconnected from a real world source of value, and information about what they are purchasing (huge numbers of crypto speculators choose cryptos based purely on the shape of the trading chart).
One large buyer can do that, but otherwise it takes a bunch of small buyers. I'm fairly familiar with what it takes to move markets of varying liquidity, because we have to operate trading bots as part of our business for inventory purposes. I've found that small trades even in illiquid markets like HBD don't tend to move the price in a way that can trigger a big price change.