I honestly have been trying to understand. I initially thought it was a timing thing. Just let the crushthrough charge for 0.5 secs, then release. But it STILL bounces off from blocks and shield.
Then I just went with a friend. I held the maul up for exactly 1 sec, released. It bounced off his shield 70% of the time.
Either this crap is bugged or something else is in play too. I have 27 strength, by the way.