Why were early AFRP A300/A310 rudders prone to delaminating?
You'd have to find someone with access to the original Airbus failure analysis to know the details, but in general it's more difficult to get good bonds with Kevlar laminates compared to graphite or glass (something to do with helpful reactive compounds on the surface of glass and graphite fibres that Kevlar lacks), and special surface treatments and epoxies are required.

I'll hazard an educated guess that Airbus engineering at the time may not have had a lot of experience with Kevlar composites and didn't appreciate the bonding issues sufficiently, and it didn't show up in testing and took the stresses of service operation to uncover it.

Nothing new there as such screw ups go... on the CRJ200, the floors were carbon honeycomb set on floor beams of aluminum.   Somebody seemed to have missed their galvanic scale class in university and didn't realize that carbon and aluminum don't like to live side by side, and areas of the floor see significant moisture from people spilling stuff, rain coming in the doors, humidity, etc.  The company ended up replacing the floor beams on the first couple hundred airplanes with titanium ones.