_You_ should be asking you how - there are lots of reasons why this could be happening and knowing which one is important if you're changing stuff.
Based on a "highly parallelizable" application performing better on 8 cores than 32, I'd guess you're running out of something else: memory or disk bandwidth.
Based on a "highly parallelizable" application performing better on 8 cores than 32, I'd guess you're running out of something else: memory or disk bandwidth.