It is possible for a window to crack due to temperature extremes.

It is also possible that the glass had been physically stressed for some reason and that could have caused the crack.

Hard to say without actually seeing the window.

Is there a reason (such as insurance) that you need to know why it cracked? or is it more curiousity?

And, unfortunately you will have to get the entire dual pane replaced. You will need to call a glass shop to help you out with it.

I'm asking because if it just cracked due to the cold then I'd hope our landlord would pay for it. If it cracked for some other reasaon (i.e. something was throw at it?) then I assume we'd have to pay for it...

Cold will not crack your glass, nor will heat crack your glass. But, temperature variations can crack glass.

If the crack started at one corner, and then ran diagonally to another corner, I would suspect that there is a stress point in the corner where the crack started and the possibilty that the glass wasn't squared-up in the sash when the window was originally glazed.

If the crack starts away from a corner then it could be another issue - again possibly related to a flaw in the glass.

I would suggest that you contact the landlord and let him know what happened - not much else you can do, unfortunately.