Curious about main pulley

[Alpine TSi]

Sorry about the pic size!!! I don't know how to resize.

First off, this isn't really a tech post, hence the reason it is in the parking lot.

On Thursday morning I heard a wierd sound from the tbelt side of my motor. Not wanting to take any chances I drove a spare vehicle until I had time to look at it today. Well I found something that was interesting. The first picture is the way it should be, and the second is what happened to it.

Is that common? Is there something that can prevent that from happening? I had the one laying around from my old Talon motor that I put on. That one seemed solid, so hopefully it will last.

I thought this was wierd, but I am still a novice when it comes to the wonders of breaking DSMs.