Congratulations on exclusive nursing your twins!
As to what causes their wakings, it could be, aside from a growth spurt, that could happen around 5.5-6 months, something developmental. Are they learning to sit up, roll over? This very often is the cause for wakings at this age. If they don't seem hungry during the day (and eat as usual) I would assume it is not a growth spurt.
Or, is there maybe some external factor, like a truck passing by at 4 am, waking them up?