Could be a mix of things. It's possible that there is some noise or something that occurs at 6.30 that wakes you up. Is there someone in your house who gets up that early?
Could be that you just can't maintain sleep for a longer time. Perhaps it's because of stress or anxiety.
Could be that you know you have to wake up soon and so your body it prepared for it. I always wake up before my alarm, for instance.
Could be that you've gotten used to waking up at 6.30 and now you're in the habit of doing so.
Given that it only occurs on weekdays, I would hazard a guess at the penultimate theory, depending on what time you have to get up (for instance, if you have to get up at 9 it probably isn't the case, but if you have to get up at 7, I would say it's a good bet.)