Good question indeed.
I'd like to know a bit more of it.
Number of hits seems to be part of the equation: I've posted some question not long ago about single sign on with Mantis. I was kind of surprised this post did become very hot very quick.
Not that is is not an interesting topic, but I think it is rather a "niche" topic.
Actually is seems that Googling "single sign on Mantis" makes the post showing up quite high in Google results, hence number of hits, hence (part) of the score regarding "hot".
But I am waiting for more details about "the equation" ...