Chain down, chain down - chain snap, snapped chain, chain break, broken chains

My SRAM chain snapped when I was pulling away at a traffic light. It looks like one of the links has just been pulled until it snapped. It had been cleaned in the last month or so, and is only about 7/8 months old. I had heard a creak every now and then when out of the saddle going uphill etc. I can't think what could have caused it to snap as I checked for stiff links, and it was not under too high tension. The only things that come to mind are;

1. Teeth missing in chainring. Shouldn't cause damage to chain though, surely? It is a fairly old part, but the teeth are not wavy.
   
2. Chain tension: slightly slacker than perfect, but wouldn't have thought that coudl cause damage.
   
3. Chain line: Wheel was v slightly off centre between chain stays, but was riding fine with no tracking issues, and chain line looked OK.