Does anybody know what could be a reason?
My suggestion is, one (or both) of the sunroof drains are plugged up, causing water to overflow into the cabin during rain. You can easily test it by retracting the sunroof and pouring a little bit of water into the track near the drain. There are usually two drains located at the front corners. If you pour the water in there and it stays stagnant, the drain is plugged. You might also already see either a small amount of water there already or the traces of where the water line was at.
If it is plugged, use a long, pliable wire and run it down the entrance to the drain. You want to be careful doing this, because you run the possibility of poking through the tubing which makes up the drain. If you bend the end of the wire over into a small loop (like a teardrop), it will make it so it is less likely to do any damage. You'll still want to be careful. If there is a small amount of water in the drain area when you do this, you'll very easily notice when the blockage is cleared as the excess water will quickly drain out.
As an aside, the reason I come to the conclusion why this may be your drain(s) is because the area you are pointing out is a typical place where water is found when the drain gets plugged. If this was the passenger side, it could just as easily be the AC drain which is plugged, but you would usually only see it get wet there during summer months when the AC is running all the time.