Next time, before you pump up the tube, push the valve in, toward the tread of the tire. Since the tube is thicker near the valve stem, it's fairly easy to get part of the tube caught between the tire bead and the rim there. Pushing it in makes sure it's all inside the tire bead.

I like to go around both sides of the tire before pumping, squeezing the bead toward the center of the tire, and making sure I don't see any tube in the gap between the side of the rim and the tire bead.

And after it's pumped up, I hold the axle and spin the tire, looking just above the rim on each side. Most tires have a line molded into the tire there, and I see if it's showing evenly all the way around. If one spot hops up, the bead may come off there (and the tire will ride unevenly, too)

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If a fold of the tube is caught between the rim and the bead, it can eventually push the bead right off the rim there, then the tube blows up like a balloon and pops.

I've seen some other posts about blowouts when the bike is just sitting around the house. You'd think the bead would come off the rim during a ride. But it must slowly creep off the rim over a period of hours.