While "most probable" is tending towards the discussion type of answer, there are two main reason for a hip swing in a biceps curl, and both have the same root cause.
Reason 1 is to create momentum in the weights, so that rather than starting from a dead stop there is motion forward and a bit up. This makes it easier to complete each rep.
Reason 2 is to counterbalance, there is enough weight out away from the body that the hips move to compensate.
Both of these are generally caused by using too much weight for ability. If you are having to "cheat" a weight to complete a rep (In pretty much any given exercise) you are not getting as much benefit as you would from a lighter weight and stricter form. There are a few examples where cheating the exercise is correct, but those are few and specific.