But I was curious if there might be any correlation between BDSM and specific childhood experiences,

An abnormally high number of bdsm'ers I have met have experienced some form of serious sexual abuse. I wonder as well if there is correlation. A significant number of those claiming abuse in my opinion seem to be inventing or exagerating the abuse. I suspect the correlation (if any) may be an indirect one. I notice that many who have been abused have no problem talking about their nightmare experience and enjoy the sympathy they receive as a result.

Perhaps some people believe that saying their wicked stepfather used to cage them in the basement and cruelly whip them makes them more easily accepted in the "community" or that they will be higher on the totem pole. Perhaps ithey are attention seekers. Attempt to discuss this correlation seems to be met with a "how dare you". Perhaps its because it opens the lid on fibbing, deception and fraud, which is a road many do not want to go down.

There must be psychological research out there on the relation between abuse and bdsm.

I cannot for the life of me understand the idea of giving up complete agency to another person

Nor can I and I dont believe it exists as such. Much of bdsm is sexual fantasy and roleplay. Its easy to exagerate or pretend that the roleplay extends into everyday real life. In many cases a bdsm couple are a husband and wife who live a normal married life outside the bedroom but will refer to themselve within the bdsm community as master slave.

The slave does not give up complete agency to another. From time to time female slaves (sub) will post about how beastly the master was by doing this, expecting this or forgettting an anniversary. Nobody will say the master is well within his rights. All will offer sympathy to the sub and agree how beastly the master is.

The escape clause is the (supposed) masters obligation to love and care for the slave. Not sure of the origin of this obligation other than it being the same as for vanilla marital relationships. It is certainly not reflected in historical slavery. This fine print means in effect the slave not only retains control but can even exert it. Consider this dialogue.

Master: We are going out to eat Chinese tonight
Slave: But I dont feel like eating Chinese.
Master: Tough, I'm the master and I say we're eating Chinese.
Slave: But you are supposed to love and care for me. You must take my feelings into consideration and I dont feel like Chinese, i prefer Italian.
Master: Well if you feel that way we'll eat Italian,

Master Slave rarely exists in real life. It is poetic license. Forget any images of Romans or southern plantations. The closest it gets to slavery is marriage of yesteryear where the man was master of the house and whilst he may whip her with his crop she would lash him with her tongue such that a happy balance was achieved.