Did a Duathlon this morning and my rear tire fell out and only stayed connected to my bike due to the chain...

Bike is a Cervelo P3 with new profile design 58/78's clinchers that I had installed at the LBS.

The race consists of 5 loops (extremely hilly course) with the last turn on the loop being sharp right and coming from downhill, thus I was braking on the way down... On my 5th loop my rear tire fell out of the track and was held on with the chain only. I pulled aside, re-attached and finished the race...

Other than clinchers being too loose, what other factors could have contributed to this? Could the downhill braking have caused the wheel to loosen? Has anyone ever seen this happen before? I re-tightened and re-adjusted the brakes and wheels when I got home and all seems OK, however wondering if there is another issue?

A few possibilities.

The first - the simplest - is that they were simply installed poorly.

Another is that the tolerance of the rims is not very good. With carbon clinchers, achieving consistent bead seat widths is more difficult than with aluminum rims. On cheaper rims, there can be pretty significant variance. So you might have gotten a set with a "wider than normal" bead.

Yet another, which could be in concert with poor rim tolerance, is that the quality of the epoxy is not very good. Cheap resins tends to do poorly when they get hot. And carbon rims get REALLY hot under braking. If the resin isn't up to handling the heat stress, the rims can get soft. And, because of that, the pressure of the tube can actually cause the sidewalls to flex, popping the bead.

Given what you describe, my guess is that it's this. When Josh Poertner was at Zipp, they were able to recreate this phenomenon regularly. They would just pump up wheels, throw them in an oven, and watch the tires blow off the bead. Josh's theory was that this even applied in many cases to blowouts in the trunk of cars or in transition.

The thing about this is, it's very hard to recreate yourself in a controlled environment. So I don't know what to tell you to do from here.

The tires survived.... below moved backwards and came off during the down hill braking? I pulled over, re-clinched and finished the race with the brake rubbing against the rim due to mis-alignment....

Define "re-clinched". I think we may have a terminology problem.

Define "re-clinched". I think we may have a terminology problem.

Yes, OP please clarify. The word "Clincher" is usually only used in reference to the type of tire, or rim that uses a "clincher" rather than "tubular" tire. The "quick-release" is what is used to hold the entire wheel on the bike. So when you say "my rear tire fell out of the track and was held on with the chain only", do you mean just the tire, or the entire wheel? And when you say "re-clinched", do you mean you tightened the quick release for the wheel, or re-installed your clincher tire?

Re-clinched = re-attached wheel... yes, 7 years in the sport and I do not know all the lingo...

No excuse. Go get this book, read it cover to cover, and practice doing wheel installation, tire changes, derailleur alignment, and brake adjustment until you can do it in the dark. Buy both screwdrivers that it takes to do these ultimately basic items of bike maintenance and setup.

The one dumb thing that I see people (not just triathletes) do is to align the rear skewer QR lever so that it sticks out backwards from the axle, because "it's more aero". That is retarded. If anything hooks on that lever, your wheel will be loose in the dropouts and could come out inadvertently, instantly negating any aero advantage you may have gained. So align the QR lever so it points between the chainstay and the seatstay (Johnny, go look that up in the book), and that way it is NOT coming undone unless you mean to undo the wheel.