what is causing that some people are submissive in sex (bdsm)?

I think porn encourages people to think more in terms of dominant and submissive sex roles. I think fetish in particular, definitely has come to be a result of mass porn consumption.

Someone always has to take a submissive role in sex.

Most often, women take the submissive role, which can be tied to biology and reproduction. Until a couples of decades ago, it wasn't even considered that women even enjoyed or found any pleasure from sexual intercourse.

However, times have changes and so have gender roles, meaning people can happily play around with submissive and dominant roles. However, some extreme sexual interests are said to be tied to the person's past, which might have involved abusive or neglectful experiences - later developing into fetishes or sexual interests.

There are hundred of speculations and reasons as to why some people have certain fetishes, fantasies and sexual interests.