I used to own a paintball shop and filled cylinders regularly. If you overfill the tank there's a good chance with any temp change or shaking/moving the burst can go. I've even seen properly filled tanks burst because people have left them in the back car window on a hot day in the sun.
This is why when filling a tank you need to chill it first. I'd add some CO2 then vent it out to insta-chill the tank, but the best method is to put it in the freezer for several hours to chill everything evenly then fill it from that temp. If you're filling with food grade CO2, they'll probably chill them via freezer to save the wasted gas.
Anyway, don't sweat it. Your tank is most likely fine. Burst disks are cheap and easy to replace and in this case it did exactly the job it was supposed to do. Have them replace it, chill, refill, then enjoy your tank as usual.
[edit] just seeing you already had it replaced. It's a shame they didn't want to admit the overfilled, but that's what must have happened. Especially if you didn't leave it near any heat source or shook it violently or anything.