I'm telling you, the beach is cursed. It reduces the IQ of everyone on it. It's the same beach where Saurfang inexplicably spared Malf who somehow survived for five minutes despite having an axe in his spinal column and bleeding out. Now in the same beach Malf inexplicably allows Nathanos to run away instead of shooting him.
This is also a handy explanation as to why Sira and Delaryn switch sides and Sylvanas burns the tree. That happened on the beach too and it made them retarded.