Trust partner's card and south's bidding

George Eliot wrote, "There are men whose presence infuses trust and reverence."

If your bridge partner is one of those, lucky you. But regardless, you must be able to rely on your partner's bids and plays. Trusting the opponents is a tad more dangerous because they might intentionally be trying to mislead you; however, most of the time, they are dependable.

This deal was played at Bridge Base Online last month. It looks so easy to defeat three no-trump, but it made seven times and failed only thrice. Why?