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.