The term "bum hungry syndrome" derives from the idea of your arse being so hungry it has actually stared to eat your pants...
BHS is frequently caused by ill fitting underwear, specific breeds of underwear cause chronic bum hungry syndrome, so much so that it may mature into "starving bum syndrome" these pants include: girl boxers, high legged briefs and the notorious laced pants. A note for those trying to avoid BHS...ironically pants purchased from the shop "BHS" (British home stores) do not cause the disease, though ones bought at Primark do (speaking from personal experience!)
In serious cases when the knicker-line of the BHS is visible from the outside of your clothing, it is at this point that the BHS must be ejected. One advises you walk quickly to an uncrowded area and do the deed, as it were.
Old man to a comrade: "dude, you think here ass it hungry?!"
Perturbed dog: "woof...that is some bad ass Bum hungry syndrome you got goin’ on there my brutha!...woof"