Several class enhancements, such as Thief Acrobat and Air Savant, grant immunity to most knockdown effects. The only exceptions that I could find were mind spells (such as Greater Command causing you to lay down), scripted attack sequence events (such as the Titan hitting the ground and causing Knockdown on a failed balance skill check), and Air Elemental knockback/knockdown effects.
Now, however, Giants using their knockdown ability seem to bypass this immunity as well. They did not bypass it in the past, although their knockback attacks were unaffected. Is this a bug or is it working as intended? Should I reset my enhancements to see if that fixes the issue?