Sounds like you stretched one of the buckles (they are like the D-rings you get on motorbike helmets) but they arn't welded shut sometimes and when you give them a really good pull one pops over the other and it comes loose
Have a good look at them and if one is bigger you can shut it again with a big pair of pliers
Otherwise are you sure you are doing it up right as it is easy to thread them wrong
Jason