Looks like something was wrong with the casing (they look like they have 4000km's on them
!!!). Maybe some shape irregularity that caused premature wear on the rubber and eventually weakness on a specific spot.
Happy for you it didn't cause anything bad.
I ride on Challenge Criteriums now on my everyday wheelset and I must say I love them. Better than my Vittoria Corsa CX or SC, who ALL had bulges aorund the valve...
Maybe your friend's "deals" come from some defect batch that shouldn't be sold, or at least shouldn't be ridden for as long as you did. I saw some Criteriums tubs sold on Ebay last year for $45. per PAIR
!!!! You get what you pay for...