It really shouldn't, but there are situations where Catholic rulers will ask the Pope to call a crusade against a specific target. If Turkmenstan is Catholic, maybe the ruler asked for a crusade against Khotan? But he didn't join it, so....
I believe they are called more or less randomly whenever you have a situation where there are no valid targets with non-zero Crusade weights. This can happen pretty easily in 1066 and later starts, e.g. if Jerusalem has either already been liberated or is otherwise invalid and the Reconquista has gone swimmingly.
I still have fond memories of a recent campaign where the Pope called a Crusade for Arabia, and I was excited by an at least marginally reasonable target and gathered my army and fleet to invade (Jerusalem was Muslim but entirely in revolt, so not eligible, and I had already conquered all of Spain for myself)...until I realized he had called it against the Seljuks, who owned exactly one province that stretched into de jure Arabia, located in the exact center of the Arabian peninsula (and able to support a whole 5k troops without attrition). Just marching there from the nearest port killed off tens of thousand poor Spaniards in a mad quest to gain the crusader trait for my zealous ruler.