Doflamingo isn't as strong as the fact that he's hyped as shit. In part because of idiot fans like the lot of you. He is strong true, but I think you all forget that the Shuchibukai - collectively - are needed by the WG to balance out the scales of power between them and the Yankou. A single member of the Shuchibukai is a child in the eyes of the Yankou and I'd say the same about two of them. It is the collective might of the group - in collaboration with the Marines - which keeps the Yankou in check. Doflamingo is a strong physical combatant yes, and his DF allows him to use "threads" to travel, ensare his prey, and even use said threads like a whip. If anyone thinks he's anything less than a joke to someone like Kaidou then you need to donate your brain to someone who might actually use it.
The reason why OP can't fathom Doflamingo being afraid of Kaidou is because OP fell into that same baseless wankdom the Doflamingo flappers have about this character. The Yankou are on an entirely different level from the Shuchibukai, the Supernovas, and pretty much everyone. We're talking about four people who more or less control the sea, and basically the political-military landscape because the WG is so cautious of them that they have to carefully plan each move they make. Sure these people have a lot of followers but it's their very presence which keeps other pirates and the WG in check. Kaidou himself single-handedly attacked Whitebead (the World's Strongest Man) at one point and in addition he has an army of zoan DF users. Yet somehow, OP can't possibly fathom that a henchman of Kaidou (or former henchman if you prefer) is afraid of Kaidou. I'd ask are you shitting me but I'm sure you aren't.