This is just a wild guess on my part, but it was hot out today. Sitting at the pump, engine off, for 5 or 10 minutes, gives things under the hood plenty of time to get as hot as the engine, since there's no cooling air coming through the grill.
If it has been a long time since you had the brake fluid flushed and replaced, it is possible that the heat boiled it, which would give you no hydraulics. Brake fluid is notoriously attractive to moisture, and the effect of moisture in it is to lower the boiling point. Many auto makers, particularly those that make sports cars, but others as well, call for brake fluid flush, bleed, and fill every couple years. Volkswagen and BMW among them. It's just part of the routine maintenance.
If this was indeed what happened to you, you are fortunate that it didn't happen while you were going down some long steep hill somewhere.
But, as others have noted, there are other possible causes, too. All of which should be checked out. We want you to be safe out there.