This just happened to me as well. Rear tyre, sudden blowout.
Apart from the mechanical causes others have mentioned, tyre carcases do occasionally fail due to flaws in the fibres used. In these days of synthetic fibres this is fortunately very rare, but it used to be far more common in the past.
During the natural fibre days there was an episode in the 1950s when Dunlop decided to switch to Egyptian cotton for economic reasons, but it turned out to be a disastrous decision. Much of their production of tyres for all vehicle types started blowing out their tyre walls, the problem traced to the cotton. It transpired that the cotton species grown in Egypt was a short fibre type which had far less tensile strength when spun so was totally unsuitable.
Until then Dunlop had totally dominated the British market with the Avon Rubber Company as the main competitor and Michelin a small player. The widespread failures of the faulty tyres soon became common knowledge, and that together with the gap in Dunlop production let in all the competitive companies we know today. It's no exaggeration to say that Dunlop are still suffering the effects today, over half a century later. For example, in 1950 most bike tyres on sale and in use were Dunlop, but I've never seen Dunlop even mentioned in this forum, it's always Kenda, Maxxis, Schwalbe, Continental etc. The same is true with cars, all the focus seems to be on Bridgestone, Firestone, Michelin, Pirelli etc. Poor Dunlop, one attempt to save some money led to one of the biggest permanent losses of market position in corporate history.
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