Luck of the draw most likely. When an STi axle breaks, it is usually a front axle. Rear snappage is pretty rare. Sounds like you may be flat foot shifting (speed shifting) and this puts god awful stress on the axles/powertrain in general. The axles are the "relative" weak point of the STi powertrain, so look at it as better than snapping a drive shaft (you will never, ever want to feel what this is like as your driveshaft mutilates your entire undercarriage) or imho, breaking the clutch.
Also, it may have just been weakend in the launch sequence and the extra bit of "hit" when nailing second was the final straw. Lots of reasons. One thing you didn't say though was what air pressure and tires you are running? All of the stickies on launching for an STi say to raise the air pressure in the tires...I usually used around 45-50 psi. This promotes wheel spin...the best result when shocking the powertrain like that.
Also, someone may say this is a good time to upgrade your axles. My advice would be to put OEM axles in. Again, if something is going to break, an axle isn't a bad thing. Much better than a tranny or a diff.
Just to be curious, the STi you bought isn't silver with a Seibon CF hood is it?