Lots of reasons, Did you check your OEM tensioner to see where the tension was at? There's a indicator that allows you to gauge if it's maxed out or not enough.
Which pulley are you running, it could have been a belt at full tension.
I remember a post just like this a year or two ago. It snapped in the same place your's did. There was some talk that the sharp 90* corners on that bracket can develop stress cracks.