Blowout on rear rim

Rims can and do wear out, but they take a decent amount of time under normal conditions. Many rims have a wear indicator (e.g. a dot or a groove) to help tell you when they wear for wear due to rim brakes. You can also have failures long term if the rim gets damaged e.g. through hits and what not.

You're using friction to wear away not just the rubber in the rim, but that also wears away the rim material, and its ability to wear away the rim may go up through grit or other wear increases.

So, it's certainly possible that the tech's explanation is right, or some other type of rim weakness occurred.

It isn't a common occurrence since most people don't ride their bikes long enough to wear out the rims, and most people who do are cognizant enough of it through their own maintenance / shop maintenance that it is pre-empted.

Yes, that’s normal. The only other possible cause (apart from wear) would be very wide tires (relative to the rim width) at high pressure which can break the rim walls.

It’s actually amazing that rims usually last more than 10Mm (10,000km or 6,200 miles), considering all the abrasive dirt and dust. Sometimes small pebbles or aluminium shards even embed themselves in the brake pad rubber.

Make sure your brake pads wear through the rim’s braking surface evenly, not just in one narrow groove.

If your rims don’t have a wear indicator you can measure remaining rim wall thickness with special calipers. Alternatively you can compare rim width with deflated vs. inflated (to a high pressure) tire. If the rim bends outwards by a considerable amount (like >0.5mm) you should probably replace it.

Rear wheels usually wear faster than front wheels because there is more dirt on the rear wheel.

On top of the other answers, your brakes probably locked because the worn rim bulged out. Then they held the rim together until the shop freed them up.

Wear from a single large hard object embedded in the brake pad can be concentrated in a single line, and rather fast. This line of damage also concentrates the stress.

You just discovered that rims wear out from braking.

To reduce this effect in the future, you can choose high quality brake pads that don't embed grit into them. The Kool Stop salmon colored ones have a different compound with iron oxide embedded in (that's where the slightly "rusty" salmon color comes from). A word of warning, though: the Kool Stop salmons in some setups have a tendency to squeal, and the squeal can be so annoying that you might prefer traditional brake pads instead. For example, on my touring bike, I use salmons only on rear (the rear brake for some reason doesn't have a tendency to squeal on this bike) and Shimano brake pads on the front (I tested salmons on the front and they squealed like hell).

I also recommend using rims that have a wear indicator. Some rims have a deep groove. However, the groove can act as a weakener in the rim (it is a stress riser), so those rims having a deep groove wear indicator usually have a lot of supporting material and structures behind the groove to prevent the stress riser from failing the rim. This extra material increases the weight of the rim. Also, if the wheel is very accurately radially trued, the groove can cause the brake pad to wear unevenly, so that the brake pads have a ridge matching the groove, thus touching even the bottom of the groove.

My favorite style of wear indicator is used in DT Swiss TK 540 rims -- the wear indicator there is a small partial hole in the rim brake track in few locations. When the partial holes disappear, you know it's time to replace the rim.