This may be known by all. But just in case, given all the kind help shared here....

I just read a post here about how to manually crank the sky light closed. And saw there all the reasons that people had to deal with that.

Looks like the common cause is the skylight rain drains were either crushed in original assembly or plugged. The innocent issue of such plugged drains can cause EXPENSIVE problems I see--like the roof opening by itself(!)--because the circuitry and motors that open and close the cloth screen and the glass roof are quickly flooded and destroyed by puddling water there. And overflow into the interior brings MOULD.

THE ANSWER?
I have been told by mechanics, just never leave your roof open when you park outside, (even the little up-lift of the rear edge setting). Especially under or even near any trees. Because, the problem mostly stems from leaves dropping or being blown into the guts of the roof only to plug those little drains.
Though I love opening the sky light when possible, I have mostly followed that advice for the last 6 years with my old XC70 and my current XC60 with no problems--so far.

Yohan