I'm going to be bold and say that there isn't a canon explanation for the slight banking effect you see on screen. However I'd quite happy to be proved wrong because it would be nice to have a solid answer that fits what we see.
Edit: Just toTo backup my assertion and to stop @DVK attacking me constantlyassuage @DVK's concerns - I have seen every movie (several times), every episode of every TV series, played the same MMO the OP has, played a number of other Star Trek games, and read a selection of the books. I cannot recall, and I have a good memory, ever seeing any explanation for banking.