pronunciation - Why is interrogative pronounced differently from interrogate

The word interrogate is usually pronounced something like in-TEH-ruh-gayte.

But we pronounce the grammatical structure (or at least, I learnt to pronounce it) 'interrogative' as in-tuh-RAU-guh-tive. Excusing my probably inconsistent renderings of the pronunciations, what explains this difference, and why is there such a difference in stress?