My guess is that it was poor hull preparation - not enough sanding. Here's my theory:

With only one fill coat applied, you did not have all the weave filled in. When you sanded it, you did not scuff up the areas that were lower than the top of the cloth. Epoxy will only adhere to cured epoxy if that cured epoxy is scuffed up (sanded). Otherwise, the surface is too hard and smooth for it to stick - just like it is easy to remove cured epoxy from a plastic mixing cup.

When you are done sanding, vacuuming, and washing the hull, look at it under a bright light. If you see any shiny spots, those are areas where the sandpaper did not touch the epoxy. Before you apply any more epoxy, you need to remove all those shiny spots. And if you did not have the cloth weave completely filled, this means you will need to sand off some of the cloth.

The good news is that it will still look fine cosmetically. However, the cloth is thinner now, so it no longer has the same strength. If you have to sand through a lot of cloth, it may be advisable to add another layer of cloth.

On my Patuxent, I taped the seam first, applied two fill coats to fill in the weave, then feathered it in, sanded it completely, and glassed the hull. I still had a couple of small spots in the tape that the sander didn't contact, and they look like tiny air bubbles under the glass. But the spots are small and few enough that it doesn't seem to be causing any problem. I definitely recommend applying the tape first.

Ken