What do you think causes Sykes to behave the way he does?

One can only speculate as to why Sykes is such an abusive, philandering piece of work. Perhaps his father acted the same way, and Sykes just thought that this was the normal way for a man to behave. Abusive men like Sykes often have backgrounds like this, when abuse—be it physical, mental, or sexual—was very much the norm.
Or perhaps Sykes had negative experiences of women when he was growing up. If this indeed was the case, then his cruel treatment of Delia can be seen as some kind of twisted act of revenge against his mother, sister, or other female relatives.
None of this is to condone Sykes's appalling behavior in any way; these are simply possible explanations for why he acts the way he does toward Delia.