Crema Catalana major fail--was it science or me?

I'd say it was science behind the failure. Cornstarch will initially appear to thicken something, but later fail if other ingredients interfere. In this case, it could be due to the fat in the egg yolks or the amount of sugar in the mixture. They prevent the starch granules from fully absorbing and swelling. It may also be due to an excessive cooking time, which makes cornstarch bonds break down as it cools. Excess fat or sugar problems can be avoided by increasing the amount of milk used, rather than using more cornstarch. As far as cooking time- that's a bit harder sometimes. You have to bake it to barely set/wobbly, and not a bit further or it can happen. It might also be that moving your ramekins didn't do them any favors either. I assume they chilled several hours, if not overnight. But movement and vibration after thickening can also cause a custard breakdown - so that is possible. To avoid the challenges with cornstarch in recipes like this I swapped to using a modified version sold as Clear Gel. It comes in a form that requires cooking as well as a somewhat "instant" thickening version. Either one resists breaking down with heat, and that makes them invaluable for canning and pie fillings which will be baked. It might be worth it to make that swap in your recipe.