In Trinity Blood, Abel Nightroad's hair sticks up when he is in vampire form. Is there any reason given either in-canon or by one of the creators?
Yes, like everyone mentioned above. This is more of a visual effect to give the viewer the sense of change in the character during that specific sequence. =)
I have read the Sunao Yoshida's novels and there is no information regarding why he gets the spiky hair. Looking in the internet I have found no interviews of Sunao Yoshida talking about it, so the only thing I can say it is just something stylish to the electric discharges he is able to do when he transforms.